Initialise XdgOutput properties on creation

Summary:
initWaylandOutput can be called before or after all properites are set.
If it's called after, we need to still have xdgOuput set correctly.

Test Plan:
Simplified Aleix's patch that avoids a no-op setScale call
Now it works.

Reviewers: #kwin, apol

Reviewed By: apol

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D20800