drm/virtio: Rewrite virtio_gpu_queue_ctrl_buffer using fenced version.

Factor function in preparation to generating scatterlist prior to locking.

Signed-off-by: David Riley <davidriley@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190911181403.40909-2-davidriley@chromium.org


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