Perf: Fix the condition to toggle PMU IRQ when CPUs are hotplugged.

Bug: 19863147

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.

(partially cherry-picked from "Perf: interrupt disable without bringing cpus up")

Change-Id: I9253d6a3bfa51b4b71d3ca51d4c306dd49ca5ef7