Rewriting vp9_set_pred_flag_{seg_id, mbskip}.

Making implementation of vp9_set_pred_flag_{seg_id, mbskip} consistent
with vp9_get_segment_id without using confusing sub(a, b) macro. Passing
mi_row and mi_col to functions explicitly instead of replying on
mb_to_right_edge and mb_to_bottom_edge.

Change-Id: I54c1087dd2ba9036f8ba7eb165b073e807d00435