Guard every remaining access to rootInfo

Summary:
For a future XFree KWin. Only remaining not guarded usages are in
Workspace::init, but that one needs to be refactored anyway for
becoming X free.

Test Plan: Compiles

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D7173