Perf: stop counters when going into hotplug

Hotplug disables the pmu irq, but if counters are
running in the window before the CPU is hotplugged off
they can overflow and generate an interrupt. Because the
interrupt is disabled, this prevents the cpu from going
down which leads to a spin loop in the cpu_down driver
which consumes 100% cpu.

Events are stopped during hotplug processing. However,
perf is hooked into the timer tick, and restarts enabled
events on every tick, even if they were stopped. Change
the event state to OFF to prevent this.

Processing for stop reads the counters, so a separate call
is no longer needed. Start processing re-enables events so
the from_idle flag is not needed during pmu_enable.

Change-Id: I27ded876315147b50d9f85c09487b107a9216641
Signed-off-by: Neil Leeder <nleeder@codeaurora.org>