cpuidle: haltpoll: Call cpuidle_poll_state_init() later

Call cpuidle_poll_state_init() only if it is needed to avoid doing
useless work.

Signed-off-by: Li RongQing <lirongqing@baidu.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>