intel/isl: Clean up some aux surface logic

intel/isl: Clean up some aux surface logic

The first check is redundant because the first thing we do in the "emit
the aux surface" section is assert that we actually have an aux_surf.
The second check involves an exclusion list of things which don't have
aux surfaces on Gen12 but an inclusion list is much simpler because it's
just "does it have MCS?".

Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4056>