thermal: core: Introduce thermal_governor_trip_crossed()

Add a wrapper around the .trip_crossed() governor callback invocation
to reduce code duplications slightly and improve the code layout in
__thermal_zone_device_update().

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>