Use QHash::value() in EffectWindowImpl::data()

Summary:
The `role` hash key is hashed twice:
* first, when calling `contains` method;
* second, when using `operator[]`.

We can do better by using [QHash::value](http://doc.qt.io/qt-5/qhash.html#value).

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: pino, davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13820