radv: use SPI_SHADER_ZERO for non-written color attachments

When colorWriteMask is 0 we can assume that this color attachment
is unused.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5434>