ac: fix exclusive scans on GFX8-GFX9

This fixes a regression introduced with scan&reduce operations
on GFX10. Note that some subgroups CTS still fail on GFX10 but
I assume it's a different issue.

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

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 2d9f401a)