drm/radeon: fix ATPX detection on non-VGA GPUs

Some newer PX laptops have the pci device class
set to DISPLAY_OTHER rather than DISPLAY_VGA.  This
properly detects ATPX on those laptops.

Based on a patch from: Pali Rohár <pali.rohar@gmail.com>

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Cc: airlied@gmail.com