drm/radeon: fix runpm handling on APUs (v4)

Don't try and runtime suspend the APU in PX systems.  We
only want to power down the dGPU.

v2: fix harder
v3: fix stupid typo
v4: consolidate runpm enablement to a single flag

bugs:
https://bugs.freedesktop.org/show_bug.cgi?id=75127
https://bugzilla.kernel.org/show_bug.cgi?id=72701



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