perf counters: protect them against CSTATE transitions

Impact: fix rare lost events problem

There are CPUs whose performance counters misbehave on CSTATE transitions,
so provide a way to just disable/enable them around deep idle methods.

(hw_perf_enable_all() is cheap on x86.)

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>