drm/nouveau: hang drm client of a master

TTM memory allocations will be hanging off the DRM's client, but the
locking needed to do so gets really tricky with all the other use of
the DRM's object tree.

To solve this, we make the normal DRM client a child of a new master,
where the memory allocations will be done from instead.

This also solves a potential race with client creation.

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