update filter mask calculation for 5 tap filter on chroma plane

when 5 tap filter(filter6) was introduced for chroma plane, it reads 6 pixel
lines/columns and modify 4 pixels. However, for the filter mask
calculation, it still use the original filter mask calculation function
for 7 tap filter(filter8), i.e, read 8 pixel lines/columns to check whether the
edge sould be filtered or not. This change updates the filter mask
calculation to make it consistent with the filering operation to only
read 6 pixels for filter mask calculation.

AWCY low latency test result show no noticable quality impact as below
RLDeblockUpdateFilterMaskBaselineLL@2018-01-02T18:42:02.782Z -> 
RLDeblockUpdateFilterMaskNewLL@2018-01-02T18:43:46.627Z

  PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
0.0206 | -0.0226 | -0.0412 |  -0.0226 | -0.0671 | -0.0398 |     0.0370

https://arewecompressedyet.com/?job=RLDeblockUpdateFilterMaskBaselineLL%402018-01-02T18%3A42%3A02.782Z&job=RLDeblockUpdateFilterMaskNewLL%402018-01-02T18%3A43%3A46.627Z

BUG=aomedia:1183

Change-Id: Iec434e48161e1bc2e67b38ea33c46e9c73774ce2