ac/llvm: adjust dmask when image stores are shrinked using the format

ac/llvm: adjust dmask when image stores are shrinked using the format

It looks like GFX10 doesn't care about dmask if it's greater than
the number of components stored but it matters on GFX8-9 (I haven't
checked older gens).

Fixes: 1b4d968106d ("ac/llvm: fix invalid IR if image stores are shrinked using the format")
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6982>