Correct code style of Client fullscreen control

Summary:
As a preperational step for future changes obey the code
styling guide in Client fullscreen functionality.

No functional changes.

Test Plan: Manually, autotests pass.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: zzag, graesslin, kwin

Tags: KWin

Maniphest Tasks: T11098

Differential Revision: https://phabricator.kde.org/D18153