Make iterating over Layer enum simple

Summary:
Having a function with a confusing comment, to iterate over the enum,
is mostly confusing. Make it boring by just moving the cast to the one
place that uses it.

Reviewers: #kwin, romangg, zzag

Reviewed By: #kwin, romangg, zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23085