[Fonts KCM] Alter DPI only on explicit user interaction

The settings are loaded after the QML is created, and once it is loaded, it becomes indeterministic which property
(checkbox checked or spinbox value) is re-evaluated first and the Binding ends up resetting the config.

BUG: 405572
FIXED-IN: 5.16.4

Differential Revision: https://phabricator.kde.org/D22397