Disable positioning in popup folderviews

Summary:
We need to use the overriden sortMode, not the model's sort mode.
The == -> === changes are just cosmetic, not related to functionality

Reviewers: hein

Reviewed By: hein

Subscribers: davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D10407