drm/amdgpu: modify amdgpu_vcn to support multiple instances

Arcturus has dual-VCN. Need Restruct amdgpu_device::vcn to support
multiple vcns. There are no any logical changes here

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>