gallium/swr: Fix array stride problem.
Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com> Tested-by: Marge Bot <mesa/mesa!4405> Part-of: <mesa/mesa!4405>
Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com> Tested-by: Marge Bot <mesa/mesa!4405> Part-of: <mesa/mesa!4405>