Adding functions with common code for superblock decoding.

Adding decode_sb_8x8 and decode_sb_4x4 with common code for superblock
decoding. Renaming decode_superblock32 to decode_sb32 and
decode_superblock64 to decode_sb64.

Change-Id: Id006d7e398b9bfa3acec4326e1e0c537ebfefdd3
1 file changed