drm/vboxvideo: Move the vboxvideo driver out of staging

The vboxvideo driver has been converted to the atomic modesetting API
and all FIXME and TODO items have been fixed, so it is time to move it out
of staging.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190304164724.10210-4-hdegoede@redhat.com