pan/bi: Extract LD_VAR sample field from ins->load_vary.interp_mode

So we can extend bi_emit_ld_vary() to support centroid and sample modes.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7472>
4 files changed