Create screens before screen edges

Summary:
Screens is used in screenEdges in a recent refactor

This didn't affect wayland as main_wayland creates screens earlier
itself. We can't move creation into main_x11 as X11's screens uses an
event filter which relies on workspace existing.

BUG: 419178

Test Plan: Restarted on X11

Reviewers: KWin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D28233