Weren't critical previously, the buffers would go away anyway.  But with
recent changes to core drm/ttm lockdep will get pissed off now, so let's
fix it.
Reported-by: NMaarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>
Acked-by: NMaarten Lankhorst <maarten.lankhorst@canonical.com>