drm/radeon/atpx: add a query for ATPX dGPU power control

The runtime pm sequence is different depending on whether or
not the platform supports ATPX dGPU power control.

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>