drm/virtio: fix prime export for vram objects

drm/virtio: fix prime export for vram objects

commit c3ca06e11174 ("drm/virtio: implement blob resources: implement
vram object") and commit 3ee2cb5e48be ("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: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20210107210743.269885-1-olvaffe@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>