Force faster updates if our last output was without a PV.

This fixes an issue with the last commit, where we could end up having
a situation like:

 0.000: New move arrives, engine is busy stopping
 0.200: Engine still hasn't responded, we force display (with no PV)
 0.201: Engine finally comes back with a PV
 1.200: Display the PV

In this case, we'd be showing no PV and score for almost a second.
Fix it by starting a new 200ms timer after the PV starts coming on,
so we'd show something at 0.401 in this case (and then 1.401 etc.).
(We don't display immediately, since it's likely to be at depth 2 or so.)