sched / idle: Move the default idle call code to a separate function

Move the code under the "use_default" label in cpuidle_idle_call()
into a separate (new) function.

This just allows the subsequent changes to be more stratightforward.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>