Problem persists in kernel 4.17-rc6 from http://kernel.ubuntu.com/~kernel-ppa/mainline/v4.17-rc6/

#uname -r
4.17.0-041700rc6-generic

#cat /proc/cmdline
BOOT_IMAGE=/vmlinuz-4.17.0-041700rc6-generic <redacted> amdgpu.cik_support=1 radeon.cik_support=0 amdgpu.dc=1

#ls /sys/class/drm
card0  card0-eDP-1  card0-HDMI-A-1  renderD128  ttm  version  <<--***NOTE no VGA***

#dmesg | grep drm
[    2.326305] [drm] radeon kernel modesetting enabled.
[    2.327644] fb: switching to radeondrmfb from EFI VGA
[    2.372539] [drm] amdgpu kernel modesetting enabled.
[    2.380846] [drm] initializing kernel modesetting (KAVERI 0x1002:0x130A 0x17AA:0x3988 0x00).
[    2.380878] [drm] register mmio base: 0xF0B00000
[    2.380880] [drm] register mmio size: 262144
[    2.380889] [drm] add ip block number 0 <cik_common>
[    2.380892] [drm] add ip block number 1 <gmc_v7_0>
[    2.380894] [drm] add ip block number 2 <cik_ih>
[    2.380897] [drm] add ip block number 3 <kv_dpm>
[    2.380900] [drm] add ip block number 4 <dm>
[    2.380903] [drm] add ip block number 5 <gfx_v7_0>
[    2.380906] [drm] add ip block number 6 <cik_sdma>
[    2.380909] [drm] add ip block number 7 <uvd_v4_2>
[    2.380911] [drm] add ip block number 8 <vce_v2_0>
[    2.404721] [drm] BIOS signature incorrect 0 0
[    2.404832] [drm] vm size is 64 GB, 2 levels, block size is 10-bit, fragment size is 9-bit
[    2.404855] [drm] Detected VRAM RAM=1024M, BAR=1024M
[    2.404858] [drm] RAM width 64bits UNKNOWN
[    2.405130] [drm] amdgpu: 1024M of VRAM memory ready
[    2.405133] [drm] amdgpu: 3072M of GTT memory ready.
[    2.405145] [drm] GART: num cpu pages 262144, num gpu pages 262144
[    2.405180] [drm] PCIE GART of 1024M enabled (table at 0x000000F400040000).
[    2.405266] [drm] Internal thermal controller without fan control
[    2.405269] [drm] amdgpu: dpm initialized
[    2.406296] [drm] Found UVD firmware Version: 1.55 Family ID: 9
[    2.406649] [drm] Found VCE firmware Version: 50.10 Binary ID: 2
[    2.407995] [drm:construct [amdgpu]] *ERROR* construct: Invalid Connector ObjectID from Adapter Service for connector index:3! type 0 expected 3
[    2.408115] [drm:construct [amdgpu]] *ERROR* construct: Invalid Connector ObjectID from Adapter Service for connector index:4! type 0 expected 3
[    2.408204] [drm:construct [amdgpu]] *ERROR* construct: Invalid Connector ObjectID from Adapter Service for connector index:5! type 0 expected 3
[    2.408324] [drm:construct [amdgpu]] *ERROR* construct: Invalid Connector ObjectID from Adapter Service for connector index:6! type 0 expected 3
[    2.420247] [drm] Display Core initialized with v3.1.38!
[    2.426796] [drm] SADs count is: -2, don't need to read it
[    2.427373] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[    2.427379] [drm] Driver supports precise vblank timestamp query.
[    2.446239] [drm] UVD initialized successfully.
[    2.561743] [drm] VCE initialized successfully.
[    2.569100] [drm] fb mappable at 0xA042A000
[    2.569106] [drm] vram apper at 0xA0000000
[    2.569109] [drm] size 8294400
[    2.569111] [drm] fb depth is 24
[    2.569113] [drm]    pitch is 7680
[    2.569233] fbcon: amdgpudrmfb (fb0) is primary device
[    2.657000] amdgpu 0000:00:01.0: fb0: amdgpudrmfb frame buffer device
[    2.678478] [drm] Initialized amdgpu 3.25.0 20150101 for 0000:00:01.0 on minor 0