drm/nouveau/ltcg: fix allocating memory as free

Allocating type=0 marks the memory as free. This allows the ltcg memory
to be allocated twice.

Add a BUG_ON in core/mm.c to prevent this ever happening again.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>