drm/amdgpu/vcn: Update vcn.cur_state during suspend

Replace vcn_v1_0_stop with vcn_v1_0_set_powergating_state during suspend,
to keep adev->vcn.cur_state update. It will fix VCN S3 hung issue.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>