Rely on output device existence
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
Maniphest Tasks: T11459
Differential Revision: https://phabricator.kde.org/D23486