Optimizing set_contexts() function.

Inlining set_contexts_on_border() into set_contexts(). The only difference
is the additional check that "has_eob != 0" in addition to
"xd->mb_to_right_edge < 0" and "xd->mb_to_right_edge < 0". If has_eob == 0
then memset does the right thing and works faster.

Change-Id: I5206f767d729f758b14c667592b7034df4837d0e