aco/ngg: Setup NGG GS.

Make it possible for ACO to recognize when to use HW NGG GS.
Also add a few notes about the various GS stages in the comments.

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