Draft: dri: Remove the __DRI_DRI2 extension from some backends

Adam Jackson requested to merge ajax/mesa:dri2-extension-cleanup into main

The __DRI_DRI2 extension really does map to client-side support for the DRI2 X11 extension, AFAICT. Nobody should be doing Vulkan with DRI2 so we can drop that from kopper, and drisw does nothing special for swaps or buffer allocation for DRI2 so we can drop that too.

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