radv: only expose storageInputOutput16 for chips with double rate fp16

radv: only expose storageInputOutput16 for chips with double rate fp16

This feature allows to use both 16-bit integers and 16-bit floats
as inputs/outputs.

This disables storageInputOutput16 on GFX8 because only GFX9+ supports
double rate packed math.

This improves consistency regarding other AMD Vulkan drivers.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Acked-by: Daniel Schürmann <daniel@schuermann.dev>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4453>