We want to skip some types of aux usages (for instance,
ISL_AUX_USAGE_HIZ when the hardware doesn't support it, or when we have
multisampling) when sampling from the surface.

Instead of checking for those cases while filling the surface state and
leaving it blank, let's have a version of aux.possible_usages for
sampling. This way we can also avoid allocating surface state for the
cases we don't use.

Fixes: a8b5ea8e

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