gallivm/tgsi: fix stream id regression

gallivm/tgsi: fix stream id regression

This broke TGSI GS shaders with llvmpipe, it wasn't looking at the
right immediates and it should be cast to an integer type.

Fixes: 163d5fde0669 (gallium/swr: Enable GL_ARB_gpu_shader5: multiple streams)
Reviewed-by: Krzysztof Raszkowski <krzysztof.raszkowski@intel.com>
Acked-by: Jan Zielinski <jan.zielinski@intel.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3949>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3949>