drm/amdgpu: add vcn decode ring support

Add the decode ring init.

Signed-off-by: Leo Liu <leo.liu@amd.com>
Acked-by: Chunming Zhou <david1.zhou@amd.com>
Acked-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>