Guard the OBMC not to use 2x2 level prediction block

Limit the minimum prediction block size for chroma component to
be 4x4 in OBMC. This resolves an enc/dec mismatch when all the
experiments are turned on.

BUG=aomedia:548

Change-Id: I3151912387943b03ef5c9aa2d10fe13d15c79cf7
1 file changed