for sub8x8 block, chroma prediciton mode is obtained from the
bottom/right mi structure of the co-located 8x8 luma block. so for
chroma plane, mi_row and mi_col should map to the bottom/right mi
structure, i.e, both mi_row and mi_col should be odd number for chroma
plane. the original implementaiton always map to the top/left mi
structure of the co-located 8x8 block. this patch fixes that.

BUG=aomedia:1087

Change-Id: Id5482c5a61828e3c13adb985bd2f0606eb05a3bd