wayland: Refactor linux-dmabuf buffer management

At the moment, the render backend provides its specific implementation
of LinuxDmaBufV1ClientBuffer. This has some of its limitations. For
example, due to the strong coupling, compositing restarts must be
handled carefully. It's hard to have a generic code path to import
dmabufs, which would be nice to have in order to unify graphics buffer
allocation across various backends; currently, it's all scattered.

To make the code simpler, this change drops the commented out YUV import
code path for now. Given that Mesa implicitly handles it, the need for
it is no longer so urgent.