zink: move 8bit index handling out of u_primconvert path

putting this through util_translate_prim_restart_ib reduces our reliance on
u_primconvert, which ideally we don't want to be using

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5912>
1 file changed