riscv: Improve averaging performance in `png_read_filter_row_avg_rvv`

Replace the two-instruction sequence (vwaddu and vnsrl) with a single
vaaddu instruction for computing the average.

The vaaddu instruction with vxrm=2 (round-toward-zero) produces
identical results to the widening add followed by narrowing shift,
but in a single operation.

Reviewed-by: Cosmin Truta <ctruta@gmail.com>
Signed-off-by: Cosmin Truta <ctruta@gmail.com>