[DRM plugin] Reorganize DrmBuffer

Split off GBM based buffers to a separate file, which gets only included,
when GBM is available.

Note, that this also removes the gbmCallback, since already before this
patch we did delete the buffers always without it.

The plan is to later use this file for via GBM directly imported Wayland
buffers as well.

Test Plan:
Tested with Gl and QPainter backends.

Reviewers: #kwin

Subscribers: kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D5179