x86, amd: Enable L3 cache index disable on family 0x15

AMD family 0x15 CPUs support L3 cache index disable, so enable
it on them.

Signed-off-by: Hans Rosenfeld <hans.rosenfeld@amd.com>
Cc: <andreas.herrmann3@amd.com>
LKML-Reference: <1295881543-572552-3-git-send-email-hans.rosenfeld@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>