intel_idle: Move 3 functions closer to their callers

Move intel_idle_verify_cstate(), auto_demotion_disable() and
c1e_promotion_disable() closer to their callers.

While at it, annotate intel_idle_verify_cstate() with __init,
as it is only used during the initialization of the driver.

No intentional functional impact.

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