Some airlines at AMS allow you to check in your bag the night before - don't know if CX is one of them, but you should be able to find this out quite easily.

If they don't then the only other option would be to leave your baggage in one of the lockers landside... which would really defeat the objective of staying airside as you would have to go back landside to retrieve your baggage in the morning.

There are three hotels landside that are within a couple of minutes' walk of the terminal building - they may be a better bet.