Fixup deprecated use of QWeakPointer::data

Summary:
At the point init is run we can be sure that our decoration->client()
pointer is valid, but we still need to get a strongRef for the duration
of our usage for Qt6

Test Plan: Compiles

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25636