[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:

1.2 scale factor: after:

No change at 1x.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

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