arm: vp9mc: Calculate less unused data in the 4 pixel wide horizontal filter

Before:                    Cortex A7      A8     A9     A53
vp9_put_8tap_smooth_4h_neon:   378.1   273.2  340.7   229.5
After:
vp9_put_8tap_smooth_4h_neon:   352.1   222.2  290.5   229.5

Signed-off-by: Martin Storsjö <martin@martin.st>