The parameters are optimized only on Y, so disable chrominance
filtering for now. Later we can extend the syntax to have
separate parameters for the chrominance, or optimize the
parameters jointly over luminance and chrominance components.

lowres: -0.676% (from -0.759%) becomes a little worse
midres: -1.837% (from -1.520%) substantial improvement
hdres: pending

Change-Id: I98d71f48de98394b05fd9036de259cb43d007614