libinput: don't use weston_config when configuring input devices

libinput: don't use weston_config when configuring input devices

Instead add callbacks to the drm and fbdev backends and pass that to
the input backens so that when a new device needs to be configured
that is called and the compositor can configure it.

Signed-off-by: Giulio Camuffo <giuliocamuffo@gmail.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>