Fix bottom ref condition for intra prediction in cb4x4
Fix bottom ref condition for intra prediction in cb4x4

Resolve a rare enc/dec mismatch issue triggered by the 4x4 chroma
block size in sub8x8 block coding.

To avoid using 2x2 chroma block size, it always uses the top-left
4x4 luma block coding information to predict the 4x4 chroma block.
The rest 3 4x4 luma blocks will be coded independently without
coding the collocated chroma blocks.

The compression performance gains are 1.45% for lowres set (down
from 2.4% of the original cb4x4 design).

Change-Id: Iff560fcab172ed4219434d5174c4d8dfbbb04135

luctrudeau/aom,GrokImageCompression/aom,luctrudeau/aom,luctrudeau/aom,smarter/aom,GrokImageCompression/aom,GrokImageCompression/aom,smarter/aom,luctrudeau/aom,GrokImageCompression/aom,GrokImageCompression/aom,luctrudeau/aom,luctrudeau/aom,smarter/aom,smarter/aom,GrokImageCompression/aom,smarter/aom,smarter/aom