When the lower bound of a transform block rate-distortion cost is
above the current best rd cost, the only possibility that this
particular coding mode will be chosen is to fall back to all skip
mode. Hence there is no need to estimate the transform block rate
cost, distortion, etc. Obtain the sum of squared distance between
the prediction and the source would be sufficient.

This speeds up the encoding process by 5% - 10%.

Change-Id: I728728c3a42aafefd34641f0be69b3e2a9b9bbb2