drm/radeon: drop explicit pci D3/D0 setting for ATPX power control

The ATPX power control method does this for you.

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>