radv/llvm: enable 16-bit storage features on GFX6-GFX7

radv/llvm: enable 16-bit storage features on GFX6-GFX7

Should allow to play Doom Eternal on GFX6-GFX7 because the
driver now supports storageBuffer16BitAccess.

It's now supported and all CTS tests pass.

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/857
Cc: 20.0 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4339>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4339>