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.

include/backend/drm/renderer.h

@@ -30,9 +30,7 @@ struct wlr_drm_surface {
 };
 
 struct wlr_drm_fb {
-	struct wlr_buffer *wlr_buf; // original buffer
-
-	struct wlr_buffer *local_wlr_buf; // GPU-local buffer
+	struct wlr_buffer *wlr_buf;
 	struct gbm_bo *bo;
 	uint32_t id;
 };