vp8: Small adjustment to cyclic_refresh max_mbs_perframe.

For 3 temporal layers, reduce somewhat the
cyclic_refresh_mode_max_mbs_perframe parameter, from 20% to ~14%.
Small increase in PSNR/SSIM metrics.

Change-Id: Ia216fa5474048f1ef7fe3db88cd60dfef2a1bf8a