UPSTREAM: 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>

Bug: https://crbug.com/993452


Change-Id: I6f4348f1cd18752f55a3daefc2c83a8ce699204d
(cherry picked from commit 32d6c2c5b5227cfc6f796bbd760dab81b05f48e5)
Signed-off-by: Kaiyi Li <kaiyili@google.com>