Refactor encode_sb_ for 4x8/8x4 partition

Deprecate set_block_index. Replace it with get_sb_index_ for
consistency with partition search and bit-stream writing/reading.

Use b_width/height_log2 instead of mi_width/height_log2, to support
4x4 resolution partition types.

Change-Id: Ic1e71981e163c669f7ea6b3c12b831c284c4a494