backend/drm: only keep track of local buffer

Stop keeping track of buffers on the parent GPU when multi-GPU is used.

This removes support for export_dmabuf on secondary GPUs, but renderer
v6 will bring this back by managing the swapchains in wlr_output instead
of the backends.

backend/drm/drm.c

@@ -630,10 +630,14 @@ static bool drm_connector_export_dmabuf(struct wlr_output *output,
 	struct wlr_drm_backend *drm = conn->backend;
 	struct wlr_drm_crtc *crtc = conn->crtc;
 
+	if (drm->parent) {
+		// We don't keep track of the original buffer on the parent GPU when
+		// using multi-GPU.
+		return false;
+	}
 	if (!drm->session->active) {
 		return false;
 	}
-
 	if (!crtc) {
 		return false;
 	}