drm/radeon: split ATRM support out from the ATPX handler (v3)

There are systems that use ATRM, but not ATPX.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=41265



V2: fix #ifdefs as per Greg's comments
V3: fix it harder

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