pan/mdg: Handle nir_tex_src_ms_index

Goes in .z for a txf_ms coordinate, same as a shadow comparator so we
reuse the impl.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5782>
1 file changed