radv: compute the number of subpass attachments correctly

Only count color attachments twice if resolves are used, also
account for the depth stencil attachment if present.

Cc: 18.0 18.1 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit d8db5986)