Use balanced model for intra prediction mode coding

Use balanced model for intra prediction mode coding

This commit replaces the previous table based intra mode model
coding with a more balanced entropy coding system. It reduces the
decoder lookup table size by 1K bytes. The key frame compression
performance is about even on average. There are a few points where
the compression performance is improved by over 5%. Most test
points are fairly close to the lookup table approach.

Change-Id: I47154276c0a6a22ae87de8845bc2d494681b95f6