[Date & Time KCM] Fix clock display with fractional scale factor

Summary:
BUG: 396936
FIXED-IN: 5.12.8

Test Plan:
1.2 scale factor, before: {F6553934}

1.2 scale factor: after: {F6553935}

No change at 1x.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18340