Make the intra rd search support 8x4/4x8

This commit allows the rate-distortion optimization of intra coding
capable of supporting 8x4 and 4x8 partition settings.

It enables the entropy coding of intra modes in key frame using a
unified contextual probability model conditioned on its above/left
prediction modes.

Coding performance:
derf 0.464%

Change-Id: Ieed055084e11fcb64d5d5faeb0e706d30268ba18