drm/radeon: use PCI_D3hot for PX systems without dGPU power control

On PX systems without dGPU power control, use PCI_D3hot.

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>