Fix error when reconnecting external display

Reconnecting an external display fails on initial
page-flip.  This occurs when the pageflip_pending flag
has been set on the connector at the time the monitor
is removed.  Once the connector's CRTC has been deallocated,
page_flip_handler cannot find the connector by its crtc_id,
so pageflip_pending will remain set.  This causes
initialization to fail when the monitor is reconnected
with the error messages:

    Skipping pageflip on output 'X'
    Failed to initialize renderer on connector 'X': initial page-flip failed

To fix this problem, clear the pageflip_pending flag
when cleaning up the connector.

backend/drm/drm.c

@@ -1677,6 +1677,7 @@ static void drm_connector_cleanup(struct wlr_drm_connector *conn) {
 		dealloc_crtc(conn);
 		conn->possible_crtc = 0;
 		conn->desired_mode = NULL;
+		conn->pageflip_pending = false;
 		wlr_signal_emit_safe(&conn->output.events.destroy, &conn->output);
 		break;
 	case WLR_DRM_CONN_DISCONNECTED: