[effects] Don't use dynamic_cast

Summary:
qobject_cast has several advantages over dynamic_cast:
* it works without RTTI;
* it's usually faster.

Also, when it comes to casting QObjects, qobject_cast is de facto
standard.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17427