Fix Kickoff compact repr size hints in panel

Summary:
The old code was copy-pasted from Kicker but the wrong code block;
it was the code path for custom button image mode, not the regular
code path.

BUG:395594

Reviewers: broulik

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D14285