drm/virtio: add virtio_gpu_vbuf_ctrl_hdr

It is a helper to return the virtio_gpu_ctrl_hdr in a vbuf.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200205181955.202485-4-olvaffe@gmail.com


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