[effects/minimizeanimation] Use interpolate helper

Summary:
That's not really "clear" what's going on with opacity values. Use
interpolate helper to improve a little bit readability.

Test Plan: Minimized/uniminized System Settings, still works as expected.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14553