Disable early exit based on distortion in lossless

In lossless coding, distortion is always 0. Early exit based on this
metric was incorrect.

This CL also changed to use best_rd instead of distortion as the metric
for easly exit as requested by Jim.

Change-Id: I8ef3e407ac03b4abc3283b273f936a68fad5c2ab
1 file changed