Don't rescale every output every time outputs change

Summary:
Every time a display gets connected/disconnected, every output will be
reinitialized. We don't really need to reset the scaling to 1 every time
we don't know, especially since the setting has been set voluntarily.

Test Plan: When I disconnect the external display, my laptop doesn't fallback to scale=1.

Reviewers: Plasma, apol

Reviewed By: apol

Subscribers: davidedmundson, ngraham, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D20521