Separates the entropy coding context models for 4x4, 8x8 and 16x16
ADST variants.

There is a small improvement for HD (hd/std-hd) by about 0.1-0.2%.
Results on derf/yt are about the same, probably because there is not
enough statistics.

Results may improve somewhat once the initial probability tables are
updated for the hybrid transforms which is coming soon.

Change-Id: Ic7c0c62dacc68ef551054fdb575be8b8507d32a8