radv: enable VK_AMD_shader_fragment_mask on GFX6-GFX7

Works fine.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Tested-by: Marge Bot <mesa/mesa!3603>
Part-of: <mesa/mesa!3603>