Perf: Toggle PMU IRQ when CPU's are hotplugged

When a CPU is hotplugged out while a perf session
is active, disarm the IRQ when the CPU is preparing
to die. This ensures that perf doesn't lock up when
it tries to free the irq of a hotplugged CPU.
Similarly, when a CPU comes online during a perf session
enable the IRQ so that perf doesn't try to disable
an unarmed IRQ when it finishes.

Change-Id: Ic4e412e5f1effae0db34a3e4b5e7e5c65faed2a0
Signed-off-by: Ashwin Chaugule <ashwinc@codeaurora.org>

Conflicts:

	arch/arm/kernel/perf_event.c
	arch/arm/mach-msm/perf_debug.c