intel: Fix broxton 2x6 way size computation

intel: Fix broxton 2x6 way size computation

This patch is undoing the changes to way size computation
in broxton 2x6, made by below commit:

Commit: 0d576fbfbe912cf3fb9ab594bb31eb58bccf2138
Author:     Anuj Phogat <anuj.phogat@gmail.com>
i965: Simplify l3 way size computations

By making use of l3_banks field in gen_device_info struct
l3_way_size for gen7+ = 2 * l3_banks.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101306
Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Tested-by: Mark Janes <mark.a.janes@intel.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>