perf_counter, x86: remove X86_FEATURE_ARCH_PERFMON flag for AMD cpus

X86_FEATURE_ARCH_PERFMON is an Intel hardware feature that does not
work on AMD CPUs. The flag is now only used in Intel specific code
(especially initialization).

[ Impact: refactor code ]

Signed-off-by: Robert Richter <robert.richter@amd.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <1241002046-8832-2-git-send-email-robert.richter@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>