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

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.

v2: Add check for partial clear too (Ken).

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