Account for 2x2 tx_size in extrabits_size

Offset the transform block size with respect to TX_4X4 when
counting the cat6 extra bit size. This accounts for the potential
use of 2x2 transform block size in cb4x4.

BUG=aomedia:405

Change-Id: Ia15ac123e4f31dc66455c3aa6925ae5e754de493
1 file changed