cpuidle: Clean up cpuidle_enable_device() error handling a bit

Do not fetch per CPU drv if cpuidle_curr_governor is NULL
to avoid useless per CPU processing.

Signed-off-by: Gaurav Jindal <gauravjindal1104@gmail.com>
[ rjw: Subject & changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>