aco/ngg: Allow NGG GS to load per-vertex GS inputs.

They work the same way as in legacy GS, so we can reuse that.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <mesa/mesa!6964>