Destroy dmabuf implementation on EGL backend going down

Summary:
We leak memory if we do not destroy the dmabuf implementation on EGL backend
going down.

Also this makes sure everything is cleaned up on shutdown.

FIXED-IN: 5.17.4

BUG: 413637

Test Plan: Compiles, settings change and shutdown ok.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D25577