backends/drm: refactor buffer management

Xaver Huglrequested to merge
work/drm-buffer-management into master

Instead of buffers being both drm framebuffers and gbm / dumb buffers, these responsibilities are now split, which makes it possible to do zero copy screen casting in the future.

Both framebuffers and gbm / dumb buffers also now always hold a valid underlying resource, which simplifies code a bit.

Edited by Xaver Hugl

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