Fix encoding failure in motion-var and cb4x4 mode
Fix encoding failure in motion-var and cb4x4 mode

This commit makes the motion-var support cb4x4 mode. It resolves
the encoding failure issue when both experiments are enabled.

BUG=aomedia:136

Change-Id: I2fa963d62cbdd24cc54d5a95d02f2dc226e6d2d0

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