thermal/sysfs: Update governors when the 'weight' has changed

Support governors update when the thermal instance's weight has changed.
This allows to adjust internal state for the governor.

Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
[ rjw: Add two empty code lines aroung the locking ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>