drm/virtio: Use dma_buf from GEM object instance

Avoid dereferencing struct drm_gem_object.import_attach for the
imported dma-buf. The dma_buf field in the GEM object instance refers
to the same buffer. Prepares to make import_attach optional.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://lore.kernel.org/r/20250414131507.566072-3-tzimmermann@suse.de

Author: tdz

drivers/gpu/drm/virtio/virtgpu_prime.c

@@ -204,16 +204,15 @@ static void virtgpu_dma_buf_free_obj(struct drm_gem_object *obj)
 {
 	struct virtio_gpu_object *bo = gem_to_virtio_gpu_obj(obj);
 	struct virtio_gpu_device *vgdev = obj->dev->dev_private;
-	struct dma_buf_attachment *attach = obj->import_attach;
 
 	if (drm_gem_is_imported(obj)) {
-		struct dma_buf *dmabuf = attach->dmabuf;
+		struct dma_buf *dmabuf = obj->dma_buf;
 
 		dma_resv_lock(dmabuf->resv, NULL);
 		virtgpu_dma_buf_unmap(bo);
 		dma_resv_unlock(dmabuf->resv);
 
-		dma_buf_detach(dmabuf, attach);
+		dma_buf_detach(dmabuf, obj->import_attach);
 		dma_buf_put(dmabuf);
 	}