drm/radeon: add a delay after ATPX dGPU power off

commit d814b24f upstream.

ATPX dGPU power control requires a 200ms delay between
power off and on.  This should fix dGPU failures on
resume from power off.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>