Rely on output device existence

Summary:
For every abstract wayland output an output device interface is created
by the backend directly after instance creation. We can therefore rely on
its existence and remove superfluous checks.

Test Plan: Relevant auto tests pass. Wayland nested and DRM sessions tested.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Maniphest Tasks: T11459

Differential Revision: https://phabricator.kde.org/D23486