drm/nv50/kms: fix pin refcnt leaks

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