Move X11 specific Workspace initialization code into dedicated method

Summary:
This is required to start KWin/Wayland without XWayland support or
delayed XWayland support.

Test Plan: Run kwin_x11 in a nested Xephyr

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7897