drm/nouveau/kms/nv50: add null check before pointer dereference

Add null check before dereferencing pointer asyc

Addresses-Coverity-ID: 1397932
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nv50_display.c

@@ -2107,7 +2107,8 @@ nv50_head_atomic_check(struct drm_crtc *crtc, struct drm_crtc_state *state)
 					asyc->set.dither = true;
 			}
 		} else {
-			asyc->set.mask = ~0;
+			if (asyc)
+				asyc->set.mask = ~0;
 			asyh->set.mask = ~0;
 		}