vpx: [loongson] fix bug in var_filter_block2d_bil_16x
vpx: [loongson] fix bug in var_filter_block2d_bil_16x

Which cause failed case:
1. MMI/VpxSubpelVarianceTest.Ref/6
2. MMI/VpxSubpelVarianceTest.Ref/7
3. MMI/VpxSubpelVarianceTest.ExtremeRef/6
4. MMI/VpxSubpelVarianceTest.ExtremeRef/7

Change-Id: I122ca20089e14ac324edd61295cf8f506e06afc8

ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx