sched/core: Create task_has_idle_policy() helper

We already have task_has_rt_policy() and task_has_dl_policy() helpers,
create task_has_idle_policy() as well and update sched core to start
using it.

While at it, use task_has_dl_policy() at one more place.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Link: http://lkml.kernel.org/r/ce3915d5b490fc81af926a3b6bfb775e7188e005.1541416894.git.viresh.kumar@linaro.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>