CHROMIUM: virtwl: add dmabuf vfd support This change supports the new dmabuf VFD type, which provides zero-copy graphics output. TEST=use linux_dmabuf protocol in sommelier BUG=chromium:837209 Change-Id: Icdc1f7bca07be3766b123074d5f47311262fcfcd Signed-off-by: David Reveman <reveman@chromium.org> Reviewed-on: https://chromium-review.googlesource.com/1030032 Reviewed-by: Zach Reizner <zachr@chromium.org>