intel: Fix bufmgr_gem->gen for gen > 4

If the pci_device's actual gen was > 4, then we stupidly set
bufmgr_gem->gen = 6. Luckily this caused no bugs, and this fix shouldn't
change any behavior, because all checks against the gen currently have one
of the forms below:
    gen == 2
    gen == 3
    gen >= 4

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>