iris: Enable fast clears on other miplevels and layers than 0.

Until now we only supported fast clear colors on the first miplevel and layer. The main reason for it is that we can't have different fast clear values at different levels/layers, since the surface state only supports one clear value.

We can, however, enable it if we make sure we only use the same value for all levels/layers, and if one of them changes, we resolve all the others. We already do that for depth fast clears so hopefully it will be fine for color fast clears too.