drm/radeon/atpx: drop forcing of dGPU power control

Now that we handle this correctly, there is no need to force
it.

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>