radv: write the alpha channel of MRT0 when alpha coverage is enabled

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109597


Cc: 18.3 19.0 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit 0d8f0962)