[effects/blur] Lookup window once when disconnecting from blurChanged

Summary:
When disconnecting from `blurChanged` signal, `(w, connection)` pair is
being looked up three times. We can do better by using `QMap` in a more
STL-like way.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13118