drm/radeon: don't allow runpm=1 on systems with out ATPX

vgaswitcheroo and the ATPX ACPI methods are required to
power down the dGPU.

bug:
https://bugzilla.kernel.org/show_bug.cgi?id=73901



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