Refactor setup_ref_mv_list

This patch eliminates the is_inside() checking for each neighouring
block in scan_row_mbmi() and scan_col_mbmi(). Instead, in
setup_ref_mv_list(), find maximum above row_offset and left col_offset
for current block, and use them to decide which above rows and left
columns to search on. This patch doesn't change bitstream. No
noticeable speedup is seen.

Change-Id: Ic4ae74412605d86e9e675f86d23de3a69c04e8f3