[platforms/drm] Refactor updateTransform

Summary:
Small intermediate refactor. For now just do the normal rotation when a flipped
transform is requested. In the future we might want to provide the possibility
to flip the output image.

Test Plan: Compiles

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D25649