Lift output enablement into Platform

Summary:
This lifts the enablement code for outputs from the DRM backend to Platform
allowing other Wayland backends in the future to use this interface as well.

To do that we also create some helper functions on Platform level and have to
spill some KWayland classes into AbstractOutput what motivates a further split
of Platform into a Wayland child class like for AbstractOutput.

Test Plan: Disabled and enabled an output in DRM session.

Reviewers: #kwin

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23545