Remove change-set null check

Summary:
It is unnecessary to check if the change-set is null when looking at the
KWayland code. Also the check would be useless at the moment anyway because
before the check we access the change-set already in the DRM backend and at
the check we would not return if it would be null.

Test Plan: Relevant auto tests pass.

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D23428