Remove unnecessary calculation in 4-tap interpolation filter

Remove unnecessary calculation in 4-tap interpolation filter

Reduces the number of rows calculated for 2D 4-tap interpolation filter
from h+7 rows to h+3 rows.
Also fixes a bug in the avx2 function for 4-tap filters where the last
row is computed incorrectly.

Performance:
           | Baseline |  Result  | Pct Gain |
bitdepth lo| 4.00 fps | 4.02 fps |   0.5%   |
bitdepth 10| 1.90 fps | 1.91 fps |   0.5%   |

The performance is evaluated on speed 1 on jets.y4m br 500 over 100
frames.

No BDBR loss is observed.

Change-Id: I90b0d4d697319b7bba599f03c5dc01abd85d13b1