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@gmail.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit f222cf3c)