Fix maximize Apply Now rule

Summary:
AbstractClient applies maximize rules by running

maximize(maximizeMode());

but because window rules are checked only in changeMaximize
implementation, the if statement prevents us from applying maximize
rules (m and maximizeMode() are equal).

Reviewers: KWin, romangg

Reviewed By: KWin, romangg

Subscribers: romangg, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D22255