drm/amdgpu/vcn: use per instance callbacks for idle work handler

Use the vcn instance power gating callbacks rather than
the IP powergating callback.  This limits power gating to
only the instance in use rather than all of the instances.

Reviewed-by: Boyuan Zhang <Boyuan.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>