[DRM plugin] Correct Atomic Mode Setting

This patch makes the AMS execution path work with the new DrmCrtc and
DrmBuffer structure and solves major issues about:

• VT switching
• DPMS
• Hot plugging
• Logout
• Memory leaks

Test Plan:
Tested with Gl and QPainter.

Reviewers: KWin

Subscribers: kwin, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D5191