radv: only expose shaderFloat16 for chips with double rate fp16

radv: only expose shaderFloat16 for chips with double rate fp16

This disables shaderFloat16 on GFX8 because only GFX9+ supports
double rate packed math.

This improves consistency regarding other AMD Vulkan drivers and
it makes no sense to enable that feature without packed math.

This also reduces performance with Wolfeinstein Youngblood if
fp16 is forced enabled on GFX8, while it's similar on GFX9.

We might re-introduce that feature in the future with ACO support
if it ends up being faster and correct.

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>