Fix new MV clamping scheme for chroma MVs.

The new scheme introduced in I68d35a2f did not clamp chroma MVs in the SPLITMV
 case, and clamped them incorrectly (to the luma plane bounds) in every other
 case.
Because chroma MVs are computed from the luma MVs before clamping occurs, they
 could still point outside of the frame buffer and cause crashes.
This clamping happens outside of the MV prediction loop, and so should not
 affect bitstream decoding.