radv: make sure to export the viewport index if FS needs it

radv: make sure to export the viewport index if FS needs it

If FS reads gl_ViewportIndex but VS doesn't export it, it should
be zero to avoid reading garbage.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/2818
Fixes: b424d49ac05 ("radv/llvm: fix exporting the viewport index if the fragment shader needs it")
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4687>