arm: vp9mc: Minor adjustments from review of the aarch64 version

arm: vp9mc: Minor adjustments from review of the aarch64 version

This work is sponsored by, and copyright, Google.

The speedup for the large horizontal filters is surprisingly
big on A7 and A53, while there's a minor slowdown (almost within
measurement noise) on A8 and A9.

                            Cortex    A7        A8        A9       A53
orig:
vp9_put_8tap_smooth_64h_neon:    20270.0   14447.3   19723.9   10910.9
new:
vp9_put_8tap_smooth_64h_neon:    20165.8   14466.5   19730.2   10668.8

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