util: simplify PIPE_TEXTURE_CUBE case in util_max_layer()

util: simplify PIPE_TEXTURE_CUBE case in util_max_layer()

For cube resources, the array_size value should be 6.  So handle
that case as we do for array texture resources.  But assert that
array_size==6 just to be safe.

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>