Use b_width_log2_ in partition type parse

Replace mi_width/height_log2 with b_width/height_log2 in partition
type parsing at bit-stream writing stage. This allows parsing
resolution at 4x4 block level and makes the 4x4/4x8/8x4 partition
coding consistent with other superblock types.

Change-Id: I7db3617ea042e0db2dc898999b0c323bff91a22f