drm/nouveau/mmu: make nvkm_vmm_ctor() static

The function nvkm_vmm_ctor() is not called outside of the file defining
it, so make it static.

Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>