thermal/core/power allocator: Use the lockless __thermal_cdev_update() function

Use the new helper function and avoid unnecessery second lock/unlock,
which was present in old approach with thermal_cdev_update().

Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210422153624.6074-4-lukasz.luba@arm.com