This happens frequently, and they make it as easy for you as they can.
My recommendation it to go to 21 first and drop luggage, and passenger, if you like.
Then drive to 2, and park there. There will be a shuttle to take you back to 21 to board the ship. It is not walkable.
Upon return from the cruise, your car will be right there, and off you go.
Those who park by 21 will need to shuttle back from 2, adding another step to the already chaotic arrival day.
Its not the best situation, but it's manageable, and should be no cause for stress. Side note, make your life easier and pay for parking when you exit the elevators in the parking garage, before you go to your car. Much quicker that way.