Summary:
In case of a failed atomic commit the previous mode should be fallen back to.
For orientation provide again a setter that DRM backend can use to set the
transform back to the last working one.

Test Plan: Compiles, fallback works, cursor still messed up somewhat (scaling issue?).

Reviewers: #kwin

Subscribers: zzag, kwin

Tags: #kwin

Maniphest Tasks: T11670

Differential Revision: https://phabricator.kde.org/D25507