iris: Enable auxiliary buffer support

This currently regresses KHR-GL4x.compute_shader.resource-texture,
but that's a pre-existing bug (https://bugs.freedesktop.org/109113)
which should be fixed up once we have fast clear support.

src/gallium/drivers/iris/iris_resource.c

@@ -584,9 +584,6 @@ iris_resource_create_with_modifiers(struct pipe_screen *pscreen,
       }
    }
 
-   // XXX: we don't actually do aux yet
-   res->aux.possible_usages = 1 << ISL_AUX_USAGE_NONE;
-
    res->aux.usage = util_last_bit(res->aux.possible_usages) - 1;
 
    const char *name = "miptree";