thermal: core: Introduce thermal_cooling_device_present()

Introduce a helper function, thermal_cooling_device_present(), for
checking if the given cooling device is in the list of registered
cooling devices to avoid some code duplication in a subsequent
patch.

No expected functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Zhang Rui <rui.zhang@intel.com>
Reviewed-by: Zhang Rui <rui.zhang@intel.com>