This was hard-coding the assumption that the block size for the
smallest TX size was also the smallest block size. This is no
longer true since fe67ed6a landed.

As a result, for TX blocks that overlapped the frame edge, it was
only measuring distortion on the upper-left 2x2 part of each 4x4
sub-block, causing the encoder to prefer larger transforms which
cause such overlap and avoid transforms which do not, causing a
regression.

This patch uses the appropriate conversion table, which fixes the
regression.

BUG=aomedia:593

Change-Id: Id253cf0f3a5252378e3f340b8350120639ff5c88