drm/nouveau/bar: modify interface to bar1 vmm mapping

Upcoming changes will remove the nvkm_vmm pointer from nvkm_vma, instead
requiring it to be explicitly specified on each operation.

It's not currently possible to get this information for BAR1 mappings,
so let's fix that ahead of time.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>