Graphics freeze

My graphics/desktop froze randomly. Below is a kernel trace at the time of the hang:

pr 7 15:35:13 timesysab kernel: [360123.223545] ------------[ cut here ]------------
Apr 7 15:35:13 timesysab kernel: [360123.223604] WARNING: CPU: 2 PID: 1552 at /build/buildd/linux-3.16.0/drivers/gpu/drm/i915/intel_display.c:3324 intel_crtc_wait_for_pending_flips+0x16c/0x180 [i915]()
Apr 7 15:35:13 timesysab kernel: [360123.223607] Modules linked in: pl2303 hid_generic hid_logitech_dj usbhid ftdi_sio hid usbserial snd_seq_dummy binfmt_misc mmc_block ses enclosure uas usb_storage ppp_mppe ppp_async crc_ccitt ctr ccm ec_sys snd_hda_codec_hdmi rfcomm bnep arc4 uvcvideo videobuf2_vmalloc videobuf2_memops snd_seq_midi snd_seq_midi_event snd_rawmidi videobuf2_core v4l2_common intel_rapl videodev x86_pkg_temp_thermal iwlmvm dm_multipath snd_hda_codec_via intel_powerclamp snd_hda_codec_generic scsi_dh coretemp mac80211 kvm_intel media kvm crct10dif_pclmul i915 crc32_pclmul ghash_clmulni_intel aesni_intel aes_x86_64 lrw gf128mul glue_helper ablk_helper cryptd snd_hda_intel snd_hda_controller snd_hda_codec joydev serio_raw snd_hwdep snd_seq iwlwifi snd_pcm rtsx_pci_ms snd_seq_device drm_kms_helper btusb cfg80211 bluetooth drm memstick 6lowpan_iphc snd_timer lpc_ich i2c_algo_bit mei_me snd mei soundcore tpm_infineon wmi video mac_hid shpchp parport_pc ppdev lp parport btrfs xor raid6_pq rtsx_pci_sdmmc ahci psmouse r8169 libahci rtsx_pci mii dm_mirror dm_region_hash dm_log
Apr 7 15:35:13 timesysab kernel: [360123.223709] CPU: 2 PID: 1552 Comm: Xorg Tainted: G W I 3.16.0-33-generic #44-Ubuntu
Apr 7 15:35:13 timesysab kernel: [360123.223712] Hardware name: System76, Inc. Gazelle Professional /Gazelle Professional , BIOS 1.03.06RS761 07/28/2014
Apr 7 15:35:13 timesysab kernel: [360123.223715] 0000000000000009 ffff880213b57c18 ffffffff8178345b 0000000000000000
Apr 7 15:35:13 timesysab kernel: [360123.223720] ffff880213b57c50 ffffffff8106fedd 0000000000000000 ffff880213c50000
Apr 7 15:35:13 timesysab kernel: [360123.223725] ffff8800d6f68210 ffff8800d6f50000 ffff8800d6f50000 ffff880213b57c60
Apr 7 15:35:13 timesysab kernel: [360123.223730] Call Trace:
Apr 7 15:35:13 timesysab kernel: [360123.223741] [<ffffffff8178345b>] dump_stack+0x45/0x56
Apr 7 15:35:13 timesysab kernel: [360123.223750] [<ffffffff8106fedd>] warn_slowpath_common+0x7d/0xa0
Apr 7 15:35:13 timesysab kernel: [360123.223756] [<ffffffff8106ffba>] warn_slowpath_null+0x1a/0x20
Apr 7 15:35:13 timesysab kernel: [360123.223789] [<ffffffffc08440fc>] intel_crtc_wait_for_pending_flips+0x16c/0x180 [i915]
Apr 7 15:35:13 timesysab kernel: [360123.223798] [<ffffffff810b9730>] ? prepare_to_wait_event+0x100/0x100
Apr 7 15:35:13 timesysab kernel: [360123.223829] [<ffffffffc0846bc3>] intel_crtc_disable_planes+0x33/0x1c0 [i915]
Apr 7 15:35:13 timesysab kernel: [360123.223836] [<ffffffff817885db>] ? __ww_mutex_lock+0x1b/0xb0
Apr 7 15:35:13 timesysab kernel: [360123.223865] [<ffffffffc0847b07>] haswell_crtc_disable+0x57/0x2d0 [i915]
Apr 7 15:35:13 timesysab kernel: [360123.223870] [<ffffffff81789232>] ? mutex_lock+0x12/0x30
Apr 7 15:35:13 timesysab kernel: [360123.223900] [<ffffffffc0848537>] intel_crtc_update_dpms+0x67/0xa0 [i915]
Apr 7 15:35:13 timesysab kernel: [360123.223931] [<ffffffffc084cbc9>] intel_connector_dpms+0x59/0x70 [i915]
Apr 7 15:35:13 timesysab kernel: [360123.223960] [<ffffffffc052fe19>] drm_mode_obj_set_property_ioctl+0x399/0x3a0 [drm]
Apr 7 15:35:13 timesysab kernel: [360123.223982] [<ffffffffc052fe50>] drm_mode_connector_property_set_ioctl+0x30/0x40 [drm]
Apr 7 15:35:13 timesysab kernel: [360123.223998] [<ffffffffc051ea4f>] drm_ioctl+0x1df/0x680 [drm]
Apr 7 15:35:13 timesysab kernel: [360123.224008] [<ffffffff810a9fb8>] ? __enqueue_entity+0x78/0x80
Apr 7 15:35:13 timesysab kernel: [360123.224016] [<ffffffff8101e16f>] ? __switch_to_xtra+0x13f/0x170
Apr 7 15:35:13 timesysab kernel: [360123.224024] [<ffffffff811f5568>] do_vfs_ioctl+0x2c8/0x4a0
Apr 7 15:35:13 timesysab kernel: [360123.224029] [<ffffffff817860cd>] ? __schedule+0x39d/0x890
Apr 7 15:35:13 timesysab kernel: [360123.224034] [<ffffffff811f57c1>] SyS_ioctl+0x81/0xa0
Apr 7 15:35:13 timesysab kernel: [360123.224040] [<ffffffff8178b46d>] system_call_fastpath+0x1a/0x1f
Apr 7 15:35:13 timesysab kernel: [360123.224043] ---[ end trace 17b39abdfbe62888 ]---

ProblemType: Bug
DistroRelease: Ubuntu 14.10
Package: linux-image-generic 3.16.0.33.34
ProcVersionSignature: Ubuntu 3.16.0-33.44-generic 3.16.7-ckt7
Uname: Linux 3.16.0-33-generic x86_64
ApportVersion: 2.14.7-0ubuntu8.2
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC1: nodeax 2645 F.... pulseaudio
 /dev/snd/controlC0: nodeax 2645 F.... pulseaudio
CurrentDesktop: Unity
Date: Tue Apr 7 15:56:45 2015
HibernationDevice: RESUME=UUID=dfedd1c5-c595-4165-aa06-003b20f8cf19
MachineType: System76, Inc. Gazelle Professional
ProcFB: 0 inteldrmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.16.0-33-generic root=UUID=fd2a1529-14bd-4e1d-bb45-13cd9cbe5651 ro quiet splash vt.handoff=7
RelatedPackageVersions:
 linux-restricted-modules-3.16.0-33-generic N/A
 linux-backports-modules-3.16.0-33-generic N/A
 linux-firmware 1.138.1
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 07/28/2014
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 1.03.06RS761
dmi.board.asset.tag: Tag 12345
dmi.board.name: Gazelle Professional
dmi.board.vendor: System76, Inc.
dmi.board.version: gazp9b
dmi.chassis.asset.tag: No Asset Tag
dmi.chassis.type: 9
dmi.chassis.vendor: System76, Inc.
dmi.chassis.version: gazp9b
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr1.03.06RS761:bd07/28/2014:svnSystem76,Inc.:pnGazelleProfessional:pvrgazp9b:rvnSystem76,Inc.:rnGazelleProfessional:rvrgazp9b:cvnSystem76,Inc.:ct9:cvrgazp9b:
dmi.product.name: Gazelle Professional
dmi.product.version: gazp9b
dmi.sys.vendor: System76, Inc.