radv/aco: Enable NGG GS by default.

ACO NGG GS now supports everything we need except streamout
(aka. transform feedback), but we don't use NGG anyway when
streamout is needed.

Also add a note to the new features txt.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <mesa/mesa!6964>