drm/amdgpu/pm: add some checks for PX

I.e., doesn't make sense to change power states or check the
temperature when the asic is powered off.

Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>