intel_idle: Move and clean up intel_idle_cpuidle_devices_uninit()

Move intel_idle_cpuidle_devices_uninit() closer to its caller,
intel_idle_init(), add the __init modifier to its header, drop a
redundant local variable from it and fix up its kerneldoc comment.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>