Get output position and scale from output device interface

Summary:
Let the data saved in OutputDeviceInterface be the single source of truth
and as low hanging fruits first do this for global position and scale.

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

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: KWin

Maniphest Tasks: T11459

Differential Revision: https://phabricator.kde.org/D23489