Disable Qt's high DPI scaling on shutdown/switch user dialogs

Summary:
They use Plasma's scaling, so in theory it should have absolutely no
effect.

In practice it has an effect of hitting a Qt bug with font rendering.

CCBUG: 366451

Test Plan: None done

Reviewers: Plasma, mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D3158