Fix disabled preference icon display when using Breeze icons

Summary:
With a monochrome icon using the toGray effect has no effect, so rather
use the icon intended for this. This makes preferred and non-preferred
elements distinguishable again.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D24868