clock: fill coeff_avg with the initial rate

In order to solve bad convergence of the initial jitter.

This will soften the aout->time_get imprecision at the beginning.

Here is a graph showing how the avg is smoothened at the beginning. The trace was done on top of !2018 (merged)

Note that this MR only doesn't solve #27023 (closed), there are still a LOT a frame drops.

Thanks Denis Charmet for the idea.