When the flag DISABLE_VARTX_FOR_CHROMA is on chroma is
constrained to always use the largest transform size
for the prediction unit size.
This is meant to simnplify the logic for transform size
selection for chroma with hopefully no loss.

Results:
lowres 30 frames, speed 1: -0.038% (a slight improvement).
lowres 30 frames, speed 0: 0.000% (noise level difference).

Change-Id: I14dd5b1983d908bd98e59b7d252e11f5755c97e6