Summary:
Get the physical size directly from the always available output device
interface instead of saving an additional copy in the abstract wayland
output class.

There is some ambiguity with orientation and naming that needs to be
cleaned up when output orientation is reworked.

Test Plan: Nested Wayland, Drm, virtual backends tested.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Maniphest Tasks: T11459

Differential Revision: https://phabricator.kde.org/D23496