softpipe: Constify sp_tgsi_sampler
softpipe: Constify sp_tgsi_sampler

Add a small inline function doing the casting - this is to make sure
we don't do a cast from some completely unrelated type. This commit
does not make tgsi_sampler parameters const in vfuncs themselves for
now - probably llvmpipe would need looking at before making such a
change.

Reviewed-by: Brian Paul <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@vmware.com>
Reviewed-by: Roland Scheidegger <7d58aee419d6f9201d75517c885374dff5e7d848@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler