[plugins/qpa] Return a fixed value logicalDpi

Summary:
Font scaling is based on the screen scale factor so we can just return
dpi of 96.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D22219