[platforms/drm] Make Atomic Mode Setting the new default

With the recent patches AMS should be stable enough for daily use. It was
tested by several people in the last week without reporting back any problems.
So make it the new default on master.

Leave an env variable for now to deactivate it, in case something bad happens.

Reviewers: KWin, graesslin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D5934