cb4x4 adds a new TX_2X2 to the TX_SIZES enum, which offsets the
indexing of blockzero_probs. Add a set of default probabilities for
this case (simply copied from the TX_4X4 probabilities).

These will only be used if chroma_2x2 is enabled (though moving the
other probabilities back to their correct offset will affect all
other sizes).

This is an encoder-only change.

AWCY results on objective-1-fast:

cb4x4-fix-reset_skip@2017-05-11T06:28:45.482Z -> cb4x4-fix-blockzero_probs@2017-05-11T06:29:26.242Z
   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.1520 | -0.2682 | -0.0972 |  -0.2342 | -0.1638 | -0.1701 |    -0.3278

Change-Id: I5594f19af2935c8a8d43105a27ad33f6222ed7f2