platforms/drm: Don't release drm buffers in EglGbmBackend

Also had a similar problem in !877 (merged): When we release the buffers that also removes the drm fbs, which in turn causes failing atomic commits that the atomic code doesn't handle at all (yet, I have something planned on top of !877 (merged)), the result is a black screen when compositing is restarted.

As the gbm surface doesn't have to be destroyed in EglGbmBackend::cleanupOutput, just let the smart pointers handle that automatically when the buffers get replaced or the DrmOutput gets deleted.