mesa/st: Don't assert !unify_interfaces in the passthrough edge flags case

mesa/st: Don't assert !unify_interfaces in the passthrough edge flags case

Technically, this is correct, and I think it's the right long-term
solution.  However, Iris has code that undoes the damage caused by
nir_lower_passthrough_edgeflags (see iris_fix_edge_flags in
iris_program.c), so it should be safe to do the lowering here.

I'm not marking this as closing mesa#4838 because I think we should move
the Iris code up to here, make a different version of the NIR pass, or
something different... to properly fix this problem.  In the mean time,
this gets a bunch of tests to stop crashing. :)

Fixes: a76ec17f12c ("mesa/st: Fix iris regression with clip distances.")
Acked-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11013>