drm/prime: 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: Anusha Srivatsa <asrivats@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250226172457.217725-11-tzimmermann@suse.de

Author: Thomas Zimmermann

drivers/gpu/drm/drm_prime.c

@@ -453,13 +453,7 @@ struct dma_buf *drm_gem_prime_handle_to_dmabuf(struct drm_device *dev,
 	}
 
 	mutex_lock(&dev->object_name_lock);
-	/* re-export the original imported object */
-	if (obj->import_attach) {
-		dmabuf = obj->import_attach->dmabuf;
-		get_dma_buf(dmabuf);
-		goto out_have_obj;
-	}
-
+	/* re-export the original imported/exported object */
 	if (obj->dma_buf) {
 		get_dma_buf(obj->dma_buf);
 		dmabuf = obj->dma_buf;