drm/virtio: fix pageflip flush

Sending the flush command only makes sense if we actually have
a framebuffer attached to the scanout (handle != 0).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Ezequiel Garcia <ezequiel@collabora.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20181219122708.4586-3-kraxel@redhat.com