thermal/debugfs: Create records for cdev states as they get used

Because thermal_debug_cdev_state_update() only creates a duration record
for the old state of a cooling device, if its new state is used for the
first time, there will be no record for it and cdev_dt_seq_show() will
not print the duration information for it even though it contains code
to compute the duration value in that case.

Address this by making thermal_debug_cdev_state_update() create a
duration record for the new state if there is none.

Fixes: 755113d7

 ("thermal/debugfs: Add thermal cooling device debugfs information")
Reported-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Tested-by: Lukasz Luba <lukasz.luba@arm.com>