powercap/dtpm: Change locking scheme

The different functions are all called through the
dtpm_create_hierarchy() which handle the mutex. The different
functions are used in this context, consequently with the lock always
held.

Remove all locks taken in the function and add the lock in the
hierarchy creation function.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20220130210210.549877-1-daniel.lezcano@linaro.org