drm/amdgpu/vcn: add common vcn sofware ring decode

Add common vcn sofware ring decode.

v2: fixed compiling error

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Christian Koenig <Christian.Koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>