drm/virtio: Fix NULL vs IS_ERR checking in virtio_gpu_object_shmem_init

Since drm_prime_pages_to_sg() function return error pointers.
The drm_gem_shmem_get_sg_table() function returns error pointers too.
Using IS_ERR() to check the return value to fix this.

Fixes: 2f2aa137 ("drm/virtio: move virtio_gpu_mem_entry initialization to new function")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20220602104223.54527-1-linmq006@gmail.comSigned-off-by: Gerd Hoffmann <kraxel@redhat.com>

drivers/gpu/drm/virtio/virtgpu_object.c

@@ -168,9 +168,9 @@ static int virtio_gpu_object_shmem_init(struct virtio_gpu_device *vgdev,
 	 * since virtio_gpu doesn't support dma-buf import from other devices.
 	 */
 	shmem->pages = drm_gem_shmem_get_sg_table(&bo->base);
-	if (!shmem->pages) {
+	if (IS_ERR(shmem->pages)) {
 		drm_gem_shmem_unpin(&bo->base);
-		return -EINVAL;
+		return PTR_ERR(shmem->pages);
 	}
 
 	if (use_dma_api) {