[server] Do not own dmabuf implementation

Summary:
The dmabuf implementation should be managed by the compositor and not by the
interface which might go away through signals while the implementation goes
away in the compositor simultaneously.

Test Plan: Compiles.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D25576