radeonsi/gfx9: use the VI codepath for clamping Z
This fixes corrupted shadows in Unigine Valley. The corruption disappeared when I stopped setting IMG_DATA_FORMAT_24_8 for depth. Cc: 17.2 <mesa-stable@lists.freedesktop.org> Reviewed-by:Nicolai Hähnle <nicolai.haehnle@amd.com> (cherry picked from commit 27fef5d5)