drm/amd/powerplay: implement a common API for dpms disablement

So that code can be shared between .hw_fini and .suspend.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>