drm/virtio: Make virtgpu_dmabuf_ops with static keyword

Fix the following sparse warning:

./virtgpu_prime.c:46:33: warning: symbol 'virtgpu_dmabuf_ops' was not declared. Should it be static?

Signed-off-by: Zou Wei <zou_wei@huawei.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1605338173-22100-1-git-send-email-zou_wei@huawei.com


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>