drm/amdgpu/vcn: update header to support dec software ring

Add macro, structure and function prototype to
support vcn dec software ring.

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>