drm/nouveau/core/client: allow creation of subclients

We want a supervisor client of NVKM (such as the DRM) to be able to
allow sharing of resources (such as memory objects) between clients.

To allow this, the supervisor creates all its clients as children of
itself, and will use an upcoming ioctl to permit sharing.

Currently it's not possible for indirect clients to use subclients.
Supporting this will require an additional field in the main ioctl.
This isn't important currently, but will need to be fixed for virt.

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