[Mesa-stable] radeonsi patch nomination for stable

[Marek Olšák]

Hi,

Please apply this patch to Mesa 17.2 and 17.3:

commit f222cf3c6d6fc5d9dee3742d20aa77cfff9c39f8
Author: Józef Kucia <joseph.kucia at gmail.com>
Date:   Sun Dec 31 10:19:15 2017 +0100

    radeonsi: fix alpha-to-coverage if color writes are disabled

    If alpha-to-coverage is enabled, we have to compute alpha
    even if color writes are disabled.

    Signed-off-by: Józef Kucia <joseph.kucia at gmail.com>
    Signed-off-by: Marek Olšák <marek.olsak at amd.com>

Thanks!

Marek