drm/nouveau/gem: fix "refcount_t: underflow; use-after-free"

we can't use nouveau_bo_ref here as no ttm object was allocated and
nouveau_bo_ref mainly deals with that. Simply deallocate the object.

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