iris: Use the linear version of the surface format during fast clears.

iris: Use the linear version of the surface format during fast clears.

Newer gens (> 9) will start doing the linear -> sRGB conversion of the
clear color for us, if we use a sRGB surface format. So let's make sure
that doesn't happen and keep the same semantics as before.

Even though the hardware could convert the clear color for us during
fast clear, that converted color is only used for sampling. For resolve,
the original color would be used (without the conversion). So we convert
it ourselves and the same converted color gets used for both sampling
and resolving, simplifying the whole logic.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>