Handle the sub8x8 chroma component at the unit of 2x2/4x2/2x4 level
and use the motion vector inherited from the luma component. This
improves the coding performance:

lowres 0.4%
midres 0.25%
hdres  0.15%

Change-Id: I34dff4218cfa3e5d55e7ed0341f36f4719389f7e