Clean up intra mode writing
Clean up intra mode writing

Refactor the intra mode writing to simplify the interactions
between cb4x4, ec_adapt, and ec_multisymbol.

Change-Id: I2fd106cd03e4f6d416865893b15292aa42b01281

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