thermal/core: Make cooling device state change private

The change of the cooling device state should be used by the governor
or at least by the core code, not by the drivers themselves.

Remove the API usage and move the function declaration to the internal
headers.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Link: https://lore.kernel.org/r/20210118173824.9970-1-daniel.lezcano@linaro.org