Making decode_modes_b function more straightforward.

Moving out decode_tokens function calls and adding decode_blocks boolean
variable. We only have to decode if eobtotal > 0, i.e. we have at least one
non-zero coefficient. Also inlining and remove vp9_set_pred_flag_mbskip
function.

Change-Id: I7be38b12ee8206faf0beea2bbf4d52be42575b03