Specify NoDisplay type

Summary:
Otherwise toBool doesn't work.
I didn't see it called anywhere as property("NoDisplay") == "true", so it
shouldn't be a problem like similar cases before

Test Plan: See upcoming patches

Reviewers: Plasma, Frameworks, davidedmundson, mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D4396