perf/x86/amd/power: Change hotplug notifier to a symmetric structure

perf/x86/amd/power: Change hotplug notifier to a symmetric structure

To simplify the hotplug mechanism move the starting callback to
online. There is no functional requirement that the cpumask bit has to
be set in the starting callback.

Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Borislav Petkov <bp@suse.de>
Cc: Huang Rui <ray.huang@amd.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: rt@linutronix.de
Link: http://lkml.kernel.org/r/20160713153334.944849172@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>