Just like for all other block modes, b_pred tokens can be read together
before starting macroblock reconstruction. This removes special cases
for b_pred in decode_macroblock() and allows to make decode_coefs_4x4()
static in detokenize.c.

While at it, remove the redundant handling and checking of plane_type
and block_index (i) in decode_coefs_4x4(). Since the function is static,
and is called only from decode_mb_tokens_4x4(), we don't need to worry
that the arguments ever go out of sync.

Change-Id: I2d415da0b51b89d0490a6b9e24cc86363c2090f7