This commit enables the cb4x4 mode to support ext-tx experiment. The
coding performance gains are:

       ext-tx   cb4x4    ext-tx + cb4x4
lowres  2.7%     2.6%      4.9%
midres  2.1%     1.2%      3.0%

Change-Id: I6c566b6073527262abcdbb1a0c6bcb8729988f3b