Disable the support of compound prediction modes for sub8x8 codking
blocks. Make the rate-distortion optimizations process account for
such constraints.

With the use 2x2 chroma prediction block, this makes the wrost case
number of inter predictors same as vp9. It affects the coding
gains by 0.35% for lowres, 0.17% for midres, and 0.08% for hdres.

The encoding speed is up by 10%.

Change-Id: Ieb2a83030676911baa403e586f1f800cbf485d81