Use b_mode_info to store the inter prediction mode of sub8x8 block,
in replacement of the use of partition_info. Remove redundant buffer
update for partition_info. For bus_cif at 2000 kbps, this seem to make
speed 0 about 1% faster.

Change-Id: Id1b3be45e75a24fb4b42335ac480c23e440978f6