encodebin: Don't try rate adjustment before the first buffer

encodebin: Don't try rate adjustment before the first buffer

With both audiorate and videorate, it seems more sensible to apply rate
adjustments after the first buffer appears. For example, with v4l2src,
there is often a small delay before the first video buffer turns up, and
this can cause a stuttery start because of videorate trying to ensure a
perfect stream.