Summary: At least one test requires the physical size to be set.

Test Plan:
Lock screen screen edges test is passing again. Without the change the corner
offset in the ScreenEdges class is not calculated correctly via the
physicalDpiX() and physicalDpiY() values of an auxilliary QWidget.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15183