drm/amdgpu: add a ring func for vcn start command

Needed for the proper command sequence for VCN.

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