[kcms/fonts] Don't set minimium DPI value

Summary:
The QML port added a minimum value for the font DPI, the old widgets
code goes went 1 to 1000.

A slight bodge is needed to make sure we start at 96 when a user first
toggles force font DPI, which used to implicitly work as a minimum was
set.

BUG: 395575

Test Plan:
Set different value, looked at xrdb -q | grep -i dpi
Cleared
set another value

Reviewers: Plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D17412