4.13-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ben Skeggs <redacted>

commit 227f66d2f9954f68375736af62ebcd73c6754d69 upstream.

Reported-by: Woody Suwalski <redacted>
Signed-off-by: Ben Skeggs <redacted>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/gpu/drm/nouveau/nv50_display.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/drivers/gpu/drm/nouveau/nv50_display.c
+++ b/drivers/gpu/drm/nouveau/nv50_display.c

@@ -3281,11 +3281,14 @@ 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) {