thermal: gov_fair_share: Use .manage() callback instead of .throttle()

The Fair Share governor tries very hard to be stateless and so it
calls get_trip_level() from fair_share_throttle() every time, even
though the number produced by this function for all of the trips
during a given thermal zone update is actually the same.  Since
get_trip_level() walks all of the trips in the thermal zone every
time it is called, doing this may generate quite a bit of completely
useless overhead.

For this reason, make the governor use the new .manage() callback
instead of .throttle() which allows it to call get_trip_level() just
once and use the value computed by it to handle all of the trips.

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