Fix non-4:2:0 chroma MV calculation for SPLITMV

Fix non-4:2:0 chroma MV calculation for SPLITMV

The previous code was somewhat vestigial for 16x16 MI units, but was
incorrect when called with chroma blocks larger than 4x4 because the
block index caused a reference to a non-existent BMI. This patch uses
the same MV for all chroma subblocks in SPLITMV mode, which is
suboptimal for non-4:2:0 subsamplings, but as SPLITMV may be removed
in the near future, will use this as a stop gap.

Change-Id: I3211cee5ccf1cfb426e5eef5353b0ce5bb92b4cd