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

commit d8ade3526b2aa0505132c404c05a38b73ea15490 upstream.

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>
[bwh: Backported to 3.2: s/ACPI_HANDLE()/DEVICE_ACPI_HANDLE()/]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>