radv: Use correct writemask for cooperative matrix ordering.
Suggested by @idr in !25994 (comment 2167879)
radv: Use correct writemask for cooperative matrix ordering.
Not expecting this to actually fix anything externally visible,
but reduces some invalid usage when the resulting vector is
not 16 elements long (e.g. the C/result matrix).
Fixes: 9df4703fbb5 ("radv: Add cooperative matrix lowering.")