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.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: pino, davidedmundson, kwin

Tags: KWin

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