drm/virtio: fix prime export for vram objects

commit 16845c5d ("drm/virtio: implement blob resources: implement
vram object") and commit c6069a02 ("drm/virtgpu: Set PRIME export
function in struct drm_gem_object_funcs") landed from different trees,
resulting in prime export never working for vram objects.

Cc: Gurchetan Singh <gurchetansingh@chromium.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NChia-I Wu <olvaffe@gmail.com>
Reviewed-by: NGurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20210107210743.269885-1-olvaffe@gmail.comSigned-off-by: NGerd Hoffmann <kraxel@redhat.com>

drivers/gpu/drm/virtio/virtgpu_vram.c

@@ -69,6 +69,7 @@ static const struct drm_gem_object_funcs virtio_gpu_vram_funcs = {
 	.close = virtio_gpu_gem_object_close,
 	.free = virtio_gpu_vram_free,
 	.mmap = virtio_gpu_vram_mmap,
+	.export = virtgpu_gem_prime_export,
 };
 
 bool virtio_gpu_is_vram(struct virtio_gpu_object *bo)