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

Add null check before dereferencing pointer asyc

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

drivers/gpu/drm/nouveau/nv50_display.c

@@ -2107,6 +2107,7 @@ nv50_head_atomic_check(struct drm_crtc *crtc, struct drm_crtc_state *state)
 					asyc->set.dither = true;
 			}
 		} else {
+			if (asyc)
 				asyc->set.mask = ~0;
 			asyh->set.mask = ~0;
 		}