drm/nouveau/kms/nv50: fix oops during DP IRQ handling on non-MST boards

Reported-by: Woody Suwalski <terraluna977@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/nouveau/nv50_display.c

@@ -3265,11 +3265,14 @@ nv50_mstm = {
 void
 nv50_mstm_service(struct nv50_mstm *mstm)
 {
-	struct drm_dp_aux *aux = mstm->mgr.aux;
+	struct drm_dp_aux *aux = mstm ? mstm->mgr.aux : NULL;
 	bool handled = true;
 	int ret;
 	u8 esi[8] = {};
 
+	if (!aux)
+		return;
+
 	while (handled) {
 		ret = drm_dp_dpcd_read(aux, DP_SINK_COUNT_ESI, esi, 8);
 		if (ret != 8) {