backend: reset EGL surface after buffer swap

This prevents GL commands to affect a previously current EGL surface
after a buffer swap.

backend/drm/drm.c

@@ -569,6 +569,8 @@ static bool drm_connector_commit(struct wlr_output *output) {
 		}
 	}
 
+	wlr_egl_make_current(&drm->renderer.egl, EGL_NO_SURFACE, NULL);
+
 	return true;
 }