drm/nouveau/core/mm: fill in holes with "allocated" nodes

The allocation algorithm doesn't expect there to be holes in the mm, which
causes its alignment/cutoff calculations to choke (and go negative) when
encountering the last chunk of a block before a hole.

The least expensive solution is to simply fill in any holes with nodes
that are pre-marked as being allocated.

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