[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.
Tested with Gl and QPainter backends.
Subscribers: kwin, KWin
Differential Revision: https://phabricator.kde.org/D5179