perf, amd: Remove the nb lock

Since all the hotplug stuff is serialized by the hotplug mutex,
do away with the amd_nb_lock.

Cc: Stephane Eranian <eranian@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>