[KCM Fonts] force need save to false during load to avoid state to be true too early

Summary:
This will resolve a bug (apply never enabled). Bug occurs (at least) when kdeglobals contains QFont serialization without styleName (old style)

BUG: 416358

Reviewers: Plasma, broulik, ervin, crossi, meven, ngraham, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27384