radv: enable VK_KHR_8bit_storage on GFX6-GFX7

radv: enable VK_KHR_8bit_storage on GFX6-GFX7

Enabling a Vulkan extension doesn't mean that all features need
to be implemented. DOOM Eternal crashes at launch if that ext
is not supported but it doesn't matter if the features are enabled
or not.

Let's enable it like we did for VK_KHR_16bit_storage.

Cc: 19.3 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/4299>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4299>