aco: fix exporting the viewport index if the fragment shader needs it

aco: fix exporting the viewport index if the fragment shader needs it

It's like the layer, it has to be exported via the pos and also
as a varying if the fragment shader reads it.

Fixes dEQP-VK.draw.shader_viewport_index.fragment_shader_*

Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4564>