radv: allow to enable VK_AMD_shader_ballot only on GFX8+

Scans aren't implemented on SI/CIK.

Cc: 19.2 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit e73d863a)