drm/crtc_helper: NULL encoder->crtc when switching encoders

- Previously the old encoder would be called during modeset and without a connector bad things happened.
Signed-off-by: NMaarten Maathuis <madman2003@gmail.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_crtc_helper.c

@@ -823,6 +823,10 @@ int drm_crtc_helper_set_config(struct drm_mode_set *set)
 		if (new_encoder != connector->encoder) {
 			DRM_DEBUG_KMS("encoder changed, full mode switch\n");
 			mode_changed = true;
+			/* If the encoder is reused for another connector, then
+			 * the appropriate crtc will be set later.
+			 */
+			connector->encoder->crtc = NULL;
 			connector->encoder = new_encoder;
 		}
 	}