drm: prune modes when output is disconnected.

When an output was disconnected, its mode list would remain.  If you later
plugged into a sink with no EDID (projector, etc), you'd inherit the mode
list from the old sink, which is not what you want.

taken from Fedora kernel
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_crtc_helper.c

@@ -95,8 +95,7 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector,
 	if (connector->status == connector_status_disconnected) {
 		DRM_DEBUG_KMS("%s is disconnected\n",
 			  drm_get_connector_name(connector));
-		/* TODO set EDID to NULL */
-		return 0;
+		goto prune;
 	}
 
 	count = (*connector_funcs->get_modes)(connector);
@@ -124,6 +123,7 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector,
 								   mode);
 	}
 
+prune:
 	drm_mode_prune_invalid(dev, &connector->modes, true);
 
 	if (list_empty(&connector->modes))