Assure wayland output interface creation path

Summary:
The Wayland and XDG output interfaces are created or destroyed when the
output is en- or disabled. This follows a clear path of dependencies and
we should not check if this is indeed the case but instead assert on it.

Test Plan: Nested Wayland and Drm sessions.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23540