debugging: Simplify printing DrmObjects through QDebug

Summary:
Print automatically a few properties as well as the type, so it's easier
to read debug information.

Test Plan: Debugged a fix with it

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D28641