libinput: make setting the same output a no-op

In the future evdev_device_set_output() will start getting called more
often, redundantly. Short-circuit the setting if the chosen output is
already set for an input device. This reduces churn in the logs.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>