[platforms/drm] Add hardware transformation API

Summary:
Planes might be able to do transformations without compositing required.
When changing the current transform try this with the primary plane. If this
fails fall back to no transformation at all through hardware and communicate
the fact and other information through some getters.

Also adds an environment variable to never do hardware transformations.

Test Plan: Compiles.

Reviewers: KWin

Subscribers: zzag, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D25906