This patch allows use of 8x8 and 4x4 ADST correctly for Intra
16x16 modes and Intra 8x8 modes when the block size selected
is smaller than the prediction mode. Also includes some cleanups
and refactoring.

Rebase.

Change-Id: Ie3257bdf07bdb9c6e9476915e3a80183c8fa005a