Since CDEF looks uses the luma direction for chroma, CDEF would have
to change significantly to support formats like 4:2:2.  The limited
use of such formats does not justify the complexity to support this,
so the simple solution is to mandate that the chroma planes aren't
filtered if subsampling_x != subsampling_y.  Most of the visual gain
is in luma, anyway.

This also means that the chroma strengths and chroma skip condition
shall not be sent if subsampling_x != subsampling_y.

BUG=aomedia:720

Change-Id: I35c184a6fe0908ae0fee1e74494b6904fa9a3c82