We are in the process of doing some updates in our kitchen and we just took down the old tile backsplash. The drywall is torn, there are some holes, and still some mastic from the old tile job. We are going to be putting in a white subway tile. Are we able to salvage what's there and scrape/patch and tile over it? Or do we need to completely rip it out, put up new drywall and then tile? Is it too far gone?