drm/nouveau: rip out nvkm_client.super

No longer required now that userspace can't touch anything that might
need it, and should fix DRM MM operations racing with each other, and
the random hangs/crashes that come with that.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>