Optimize 16x16 dequant and idct

As suggested by Yaowu, simplified 16x16 dequant and idct. In decoder,
after detoken step, we know the number of non-zero dct coefficients
(eobs) in a macroblock. Idct calculation can be skipped or simplified
based on eobs, which improves the decoder performance.

Change-Id: I9ffa1cb134bcb5a7d64fcf90c81871a96d1b4018