Optimized the quantization function by making it a two-pass
process. The first pass does a quick checking of the transform
coefficients against the base ZBIN, and only keep the good
enough set of coefficients for quantization. A skipping
check is added. If all coefficients are within the base ZBIN, no
quantization is needed. The second pass is the actual quantization
pass, which only processes the coefficient subset determined
in first pass. This reduces the computation. Furthermore, an
alternitive method is used for large transform size, which often
has sparse nonzero quantized coefficients.

Overall, the encoder speedup is about 4%. The quantization function
itself gets 20% faster.

Change-Id: I3a9dd0da6db030260b6d9c314a9fa48ecae89f22