cpuidle: Remove redundant check in cpuidle_switch_governor()

gov has already been NULL checked at the beginning of
cpuidle_switch_governor, so remove redundant check.

While at it, use pr_info() instead printk() to address the following
checkpatch warning:

WARNING: Prefer [subsystem eg: netdev]_info([subsystem]dev, ... then
dev_info(dev, ... then pr_info(...  to printk(KERN_INFO ...

Signed-off-by: Yu Liao <liaoyu15@huawei.com>
[ rjw: Subject and changelog edits, added empty line after if () ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>