Fix a regression preventing previews to be resized

Summary: e6c1b97d 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