thermal: power allocator: change the 'k_*' always in estimate_pid_constants()

The PID coefficients should be estimated again when there was a change to
sustainable power value made by user. This change removes unused argument
'force' and makes the function ready for such updates.

Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Reviewed-by: Ionela Voinescu <ionela.voinescu@arm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20201124161025.27694-4-lukasz.luba@arm.com