radv: replace discard with demote for Quantic Dream games

radv: replace discard with demote for Quantic Dream games

Detroit: Become Human uses dFdx/dFdy immediately after a quad-divergent
discard, which can cause the image to become white.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Cc: <mesa-stable@lists.freedesktop.org>
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3212
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5991>