drm/nouveau/subdev: remove nvkm_subdev.mutex

There's not really any nice way to assign the lock classes when we split
subdev indices into type+inst, and saves a few bytes in the structs when
a subdev has no need for it.

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