radv: fix alpha-to-coverage when there is unused color attachments

When alphaToCoverage is enabled, we should always write the alpha
channel of MRT0 if it's unused. This now matches RadeonSI.

This fixes the new CTS:
dEQP-VK.pipeline.multisample.alpha_to_coverage_unused_attachment.samples_*.alpha_invisible

Cc: 19.0 19.1 <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 91aa25f4)