cpuidle: Wrap cpu_online_mask usage with preempt_disable

Make sure preemption is disabled before we read the cpu_online_mask
to ensure that no CPUs are hotplugged out after. If we do not have
preemption disabled when we read the online mask, we run the risk of
sending an IPI to an offline CPU resulting in a BUG().

Change-Id: I349cfd0ff3d269f3d544b458eaad27b627470c09
Signed-off-by: Kyle Yan <kyan@codeaurora.org>