Correct zbinboost lookup for 8x8 quantizer

The commit fixed a problem where 8x8 regular quantizer was using the
4x4 zbinboost lookup table that only has 16 entries at each Q. The
commit assigned a uniform zbin boost value for all cases that there
are more than 16 consective zeros. The change only affects MBs using
8x8 transform. The fix has a slightly positive impact on quality.

Test results:
http://www.corp.google.com/~yaowu/no_crawl/hd_fixzbinb.html
(avg psnr: .26% glb psnr: .21% ssim: .28%)

Results on cif size clip are also positive even though gain is smaller
http://www.corp.google.com/~yaowu/no_crawl/derf_fixzbinb.html

Change-Id: Ibe8f6da181d1fb377fbd0d3b5feb15be0cfa2017