Probook 4540s with 12.04.3 3.8.0-29-generic x86_64 and
00:02.0 VGA compatible controller: Intel Corporation 3rd Gen Core processor Graphics Controller (rev 09)
01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Thames [Radeon HD 7550M/7570M/7650M]
and acpi_backlight=vendor added to boot kernel parameter backlight and its fn buttons works fine.
Probook 4540s with 12.04.3 3.8.0-29-generic x86_64 and vendor added to boot kernel parameter backlight and its fn buttons works fine.
00:02.0 VGA compatible controller: Intel Corporation 3rd Gen Core processor Graphics Controller (rev 09)
01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Thames [Radeon HD 7550M/7570M/7650M]
and acpi_backlight=