i965/miptree: Avoid -Wswitch for the Gen12 aux modes

Avoid the compiler warnings for the new enums that will be introduced in
a future commit.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

src/mesa/drivers/dri/i965/intel_mipmap_tree.c

@@ -1626,6 +1626,9 @@ intel_miptree_alloc_aux(struct brw_context *brw,
       aux_surf_ok =
          isl_surf_get_ccs_surf(&brw->isl_dev, &mt->surf, &aux_surf, 0);
       break;
+
+   default:
+      unreachable("Invalid aux usage");
    }
 
    /* We should have a valid aux_surf. */