oprofile/x86: make AMD IBS hotplug capable

Current IBS code is not hotplug capable. An offline cpu might not be
initialized or deinitialized properly. This patch fixes this by
removing on_each_cpu() functions. The IBS init/deinit code is executed
in the per-cpu functions model->setup_ctrs() and model->cpu_down()
which are also called by hotplug notifiers. model->cpu_down() replaces
model->exit() that became obsolete.

Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Robert Richter <robert.richter@amd.com>