drm/radeon: handle non-VGA class pci devices with ATRM

Newer PX systems have non-VGA pci class dGPUs.  Update
the ATRM fetch method to handle those cases.

bug:
https://bugzilla.kernel.org/show_bug.cgi?id=75401



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Cc: stable@vger.kernel.org