vp10: fix 4:2:2 chroma MVs for 8x4/4x4 blocks.

In vp9, the bottom MV would be the average of the topright and
bottomleft luma MV (instead of the bottomleft/bottomright luma MV).

See issue 993.

Change-Id: Ic91c0b195950e7b32fc26c84c04788a09321e391
2 files changed