Reduce bmi buffer length from 16 to 4

This commit removes the use of bmi_ in the first-pass encoding by
forcing encode_intra4x4block_ to use DC_PRED, followed by DCT_DCT
only, as John suggested. This makes the need for bmi buffer only
up to 4 entries, instead of 16.

Change-Id: I3410007dfae789ee46a09ae20c39d3ce3c7954aa