UPSTREAM: 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


(cherry picked from commit 6a01d277ac611663e8e810177b50d16aa6c2a915)
Signed-off-by: Greg Hartman <ghartman@google.com>
BUG: 139386237
Change-Id: I9861c9e9c57c23fa2f6c67f0d5f6aac56bae5a4a