llvmpipe: remove backwards dependency from tilecache to llvmpipe

llvmpipe: remove backwards dependency from tilecache to llvmpipe

The tile cache is a utility, it shouldn't know anything about the
entity which is making use of it (ie llvmpipe).

Remove llvmpipe parameter to all the tilecache function calls, and
also remove the need to keep a llvmpipe pointer in the sampler structs.