Merge horiz. and vert. passes in HBD Neon 2D avg convolution

The current Neon approach to 2D convolution is:
1) Filter horizontally, storing to an intermediate buffer.
2) Filter vertically, average with the dst block and store the final
   output.

This patch merges the two phases for high bitdepth 2D convolution to
avoid the storing and re-loading from the intermediate buffer. This
provides a small gain (<5%) for large block sizes but the benefit
increases for small block sizes - as the proportion of compute to
memory access decreases. These effects are amplified further when
considering little (in-order) core performance.

Change-Id: I84f1cafcfbbfa48b2cfe4b20881da9c4bc3b56ac

Author: jwright-arm