drm/radeon: Only prompt for enabling PAT when we'd allow write-combining

No use bothering users about this for whom we disable write-combining for
other reasons anyway.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>