For 1 pass vbr mode.
Increase the gf interval for case where average Q is close to
max and high overshoot is detected.

Small increase in overall avg_psnr/sssim metrics (~0.2/0.1%) for ytlive,
but improves the low-end (low bitrate) for several clips (less overshoot).

Change-Id: Ifba40f25b4861b2e0d9832c82d5359a6a3dce9f2