drm/amdgpu: skip pasid mapping for second mmhub on Arcturus

There's no LUT register for second mmhub to convert pasid since it has no ATC.

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>