vp9 svc: SVC encoder speed up.

vp9 svc: SVC encoder speed up.

Bias towards base_mv and skip 1/4 pixel motion search when using base mv.
2~3% speed up for 2 spatial layers, 3~5% speed up for 3 spatial layers.
PSNR loss:
(2 layers) 0.07dB for gips_stationary, 0.04dB for gips_motion;
(3 layers) 0.07dB for gips_stationary, 0.06dB for gips_motion.

Change-Id: I773acbda080c301cabe8cd259f842bcc5b8bc999