iris: Manually apply fast clear color channel overrides.

iris: Manually apply fast clear color channel overrides.

At the fast clear time, the only swizzle we have available is actually
the identity swizzle (which we use for most rendering). So the call to
swizzle_color_value() becomes simply a no-op, and doesn't properly zero
out the unused channels.

We have to manually override those channels.

Fixes: a8b5ea8ef015ed4a "iris: Add function to update clear color in surface state."
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>