Separate the decoding process of 4x4 block based coding (both intra
and inter) from decode_mb and move it into decode_atom_. This allows
to further move the rest per 16x16 block decoding of decode_mb into
decode_sb, and hence eventually deprecating decode_mb when SB8X8 is
enabled.

Change-Id: I678cb8007d8a57b792d7a23020edb0c74fbf4237