Summary:
With the new Output class we can set the gamma directly here. This is also
a stepping stone to adjust individual output gamma adjustment later on.

This means any future backend, which aims to support the color correction
frontend needs to use the Output class.

Test Plan: Night Color test still passes.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D11803