Perf: interrupt disable without bringing cpus up

Refactor the interrupt disabling so that interrupts
are disabled when a cpu is hotplugged out, even if
there are no perf events on that cpu, but it holds
the PMU irq. This means that when perf ends it does
not have to re-enable an offline cpu just to disable
its irq.

Change-Id: Id0e052e4386173e107abe0eb6e80cb130d92ec24
Signed-off-by: Neil Leeder <nleeder@codeaurora.org>