iris: Store the L3$ configs in the screen

iris: Store the L3$ configs in the screen

We only calculate them based on device info and never change them so
this seems like a reasonable place to put them.  We could also put them
in the context, but that's not accessible from iris_init_*_context.

Cc: "20.0" mesa-stable@lists.freedesktop.org
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3454>