drm/amdgpu: fix tonga smu resume

Need to make sure smu buffers are pinned on resume.  This
matches what Fiji does.

Cc: stable@vger.kernel.org
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Ken Wang <Qingqing.Wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>