aco: do not use ds_{read,write}2 on GFX6

aco: do not use ds_{read,write}2 on GFX6

According to LLVM, these instructions have a bounds checking bug.
LLVM only uses them on GFX7+.

This fixes broken geometry in Assassins Creed Origins.

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2489
Fixes: 4a553212fa1 ("radv: enable ACO support for GFX6")
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3746>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3746>