[platforms/drm] Refactor updateTransform
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
Differential Revision: https://phabricator.kde.org/D25649