radv/gfx10: fix implementation of exclusive scans

This implementation is loosely based on ROCm.
https://github.com/RadeonOpenCompute/ROCm-Device-Libs/blob/master/ockl/src/wfredscan.cl



This fixes dEQP-VK.subgroups.arithmetic.*.subgroupexclusive* on GFX10.

Fixes: 227c29a8 ("amd/common/gfx10: implement scan & reduce operations")
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit c9aa8439)
Conflicts resolved by Dylan Baker