perf_counter, x86: Check old-AMD performance monitoring support

AMD supports performance monitoring start from K7 (i.e. family 6),
so disable it for earlier AMD CPUs.

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Cc: Robert Richter <robert.richter@amd.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <1244714289.6923.0.camel@ht.satnam>
Signed-off-by: Ingo Molnar <mingo@elte.hu>