[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