radv: fix setting CB_SHADER_MASK for dual source blending

CB_SHADER_MASK was computed without the second color buffer
format which looks totally wrong to me.

While we are at it, copy a comment from RadeonSI.

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 e9316fdf)