drm/nouveau/mmu: qualify vmm during dtor

If the BAR initialization failed it may leave the vmm structure in an
unitialized state, leading to a null-pointer-dereference when the vmm is
dereferenced during teardown.

Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>