fix(kcm): avoid recursive OpacityMask which leads to teardown crash

Summary:
In the current code OpacityMask's source is the parent which contains
itself, which doesn't make sense. Docs explicitly say this. [1]

This leads to a crash in teardown.

A quick reshuffle fixes it.

BUG: 419625

[1] https://doc.qt.io/qt-5/qml-qtgraphicaleffects-opacitymask.html#source-prop

Test Plan:
Found reproducible steps (see bug)
No longer crashes

Reviewers: romangg

Reviewed By: romangg

Subscribers: romangg, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D29340