gallivm: pass jit_context pointer through to sampling

gallivm: pass jit_context pointer through to sampling

The callbacks used for getting the dynamic texture/sampler state were using
the jit_context from the generated jit function. This works just fine, however
that way it's impossible to generate separate functions for texture sampling,
as will be done in the next commit. Hence, pass this pointer through all
interfaces so it can be passed to a separate function (technically, it would
probably be possible to extract this pointer from the current function instead,
but this feels hacky and would probably require some more hacks if we'd use
real functions instead of inlining all shader functions at some point).
There should be no difference in the generated code for now.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>