Debug output for platforms

Summary: Mostly so DRM can report if it has AMS or not

Test Plan:
Ran on my DRM setup, printed debug.
Spawned nested compositor, printed debug on that

Reviewers: #plasma, graesslin

Reviewed By: #plasma, graesslin

Subscribers: plasma-devel, kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D8702