change to enable encoder to clamp UV motion vector

This is to avoid a rare encoder/decoder mismatch for MB using SPLITMV
mode. In decoder, the UV mv can be determined to need clamp, but the
flag is never set in encoder motion vector selection process, and the
clamp is not done in encoding in encoder.

Change-Id: I60520d3f790354c7855dadf03f0978ea9b77e2c0