Summary:
Make it more explicit what the relation is between Wayland and XDG objects
existing and enablement:

The ouput is enabled if and only if Wayland and XDG output objects exist.

We can simplify the code by replacing checks on the outputs with checking
the current enablement value.

Test Plan: Wayland nested and virtual backends.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23553