babl: adjust search depth for fishes in first pass

babl: adjust search depth for fishes in first pass

Search first only to a depth of 2, and accept the fastest found if any
faster than reference is found, if not search to depth 5. We lose out in
the unlikely to be frequent cases where 2-step babl fishes exist and a 3
step fish is faster. And gain the fishes where 4 step is faster than 3
(which was the old first pass search depth).