llvmpipe: stop creating threads in screen init

Closed Mike Blumenkrantz requested to merge zmike/mesa:lp_threads into master

When created from lavapipe for enumerating devices, this spawns a ton of threads which persist even if llvmpipe will never be used, so instead defer all thread creation until a context is created.

@sroland I'd like to get this in before the branchpoint on Wednesday, and @airlied is on holiday, so I think this is you.

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