drm/amdgpu: skip suspend/resume on DRM_SWITCH_POWER_DYNAMIC_OFF

Missed this case previously. No need to do anything if the
device is already off.  runtime pm will handle it.

Acked-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>