Set mode in AbstractOutput

Summary:
Set the mode in AbstractOutput and call into the plugin
for final change on hardware.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Maniphest Tasks: T10016

Differential Revision: https://phabricator.kde.org/D16786