Fix a regression preventing previews to be resized
Summary: e6c1b97d67f6b6c6d4ad935db14241b041b3fca4 introduced a regression preventing previews to be properly resized.
Test Plan: Manual : resizing the information panel and the window
Reviewers: Dolphin, ngraham
Reviewed By: Dolphin, ngraham
Subscribers: kfm-devel
Tags: Dolphin
Differential Revision: https://phabricator.kde.org/D22473