drm/amdgpu: add 2rd VCN instance doorbell support

Sienna_Cichlid have 2 VCN instances, using different register for range

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