i965/glk: Add l3 banks count for 2x6 configuration

2x6 configuration with pci-id 0x3185 has same number of
banks (2) as 3x6 configuration (pci-id 0x3184).

Reported-by: Clayton Craft <clayton.a.craft@intel.com>
Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Tested-by: Clayton Craft <clayton.a.craft@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: eb23be1d "i965: Add and initialize l3_banks field for gen7+"
Cc: Francisco Jerez <currojerez@riseup.net>
(cherry picked from commit 0748383a)