thermal/governors: Group the thermal zone lock inside the throttle function

The thermal zone lock is taken in the different places in the
throttling path.

At the first glance it does not hurt to move them at the beginning and
the end of the 'throttle' function. That will allow a consolidation of
the lock in the next following changes.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20220805153834.2510142-3-daniel.lezcano@linaro.org