i965/l3: Add explicit way size calculation for bxt

i965/l3: Add explicit way size calculation for bxt

There should be no functional change here because Broxton and CHV are
both gt1. Without this code however, it might seem like broxton support
is missing.

While here, put the gt1 check in front to hopefully short-circuit the
condition for the mobile cases.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>