thermal: trip: Trigger trip down notifications when trips involved in mitigation become invalid

When a trip point becomes invalid after being crossed on the way up,
it is involved in a mitigation episode that needs to be adjusted to
compensate for the trip going away.

For this reason, introduce thermal_zone_trip_down() as a wrapper
around thermal_trip_crossed() and make thermal_zone_set_trip_temp()
call it if the new temperature of the trip at hand is equal to
THERMAL_TEMP_INVALID and it has been crossed on the way up to trigger
all of the necessary adjustments in user space, the thermal debug
code and the zone governor.

Fixes: 8c69a777

 ("thermal: core: Fix the handling of invalid trip points")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>