drm/nouveau/kms: Fix memory leak in nv50_mstm_del()

Noticed this while working on redoing the reference counting scheme in
the DP MST helpers. Nouveau doesn't attempt to call
drm_dp_mst_topology_mgr_destroy() at all, which leaves it leaking all of
the resources for drm_dp_mst_topology_mgr and it's children mstbs+ports.

Fixes: f479c0ba4a17 ("drm/nouveau/kms/nv50: initial support for DP 1.2 multi-stream")
Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: <stable@vger.kernel.org> # v4.10+
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/dispnv50/disp.c

@@ -1264,6 +1264,7 @@ nv50_mstm_del(struct nv50_mstm **pmstm)
 {
 	struct nv50_mstm *mstm = *pmstm;
 	if (mstm) {
+		drm_dp_mst_topology_mgr_destroy(&mstm->mgr);
 		kfree(*pmstm);
 		*pmstm = NULL;
 	}