drm/gem-shmem: 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/20250226172457.217725-6-tzimmermann@suse.de

Author: Thomas Zimmermann

drivers/gpu/drm/drm_gem_shmem_helper.c

@@ -339,10 +339,10 @@ int drm_gem_shmem_vmap(struct drm_gem_shmem_object *shmem,
 	int ret = 0;
 
 	if (drm_gem_is_imported(obj)) {
-		ret = dma_buf_vmap(obj->import_attach->dmabuf, map);
+		ret = dma_buf_vmap(obj->dma_buf, map);
 		if (!ret) {
 			if (drm_WARN_ON(obj->dev, map->is_iomem)) {
-				dma_buf_vunmap(obj->import_attach->dmabuf, map);
+				dma_buf_vunmap(obj->dma_buf, map);
 				return -EIO;
 			}
 		}
@@ -405,7 +405,7 @@ void drm_gem_shmem_vunmap(struct drm_gem_shmem_object *shmem,
 	struct drm_gem_object *obj = &shmem->base;
 
 	if (drm_gem_is_imported(obj)) {
-		dma_buf_vunmap(obj->import_attach->dmabuf, map);
+		dma_buf_vunmap(obj->dma_buf, map);
 	} else {
 		dma_resv_assert_held(shmem->base.resv);