Port away from deprecated qVariantFromValue

Summary:
qVariantFromValue is deprecated since Qt 5.14. It's advised to use
QVariant::fromValue instead.

Test Plan: Compiles.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D24042