Force bsize for UV in I4X4 and SPLITMV

Use 4x4 block coding for UV components arbitrarily in I4X4_PRED and
SPLITMV coding modes. This is a temporary solution to enable
bit-stream support for recursive partition down to 4x4 block size.
Will separate the functionalities of 4x4 block coding rate-distortion
out from those of superblocks.

Change-Id: I03dc15d5897014f175f3f2c91e9b266091d56797