The important thing you should be concerned about is the controller of the LCD and touchscreen. There is an abundance of C libraries (not free) for that task. A quick google got me these results: Simplify Technologies and Ramtex.
If you want to find something open source, then start from your controller's type and search embedded devices forums (even if it isn't ARM, you could easily port C code). Some suggestions:
Also, some kit manufactures offer an SDK (both with and without Linux) with their boards. Purchasing a board usually gives you the license to use the code. Search for development boards with the same LCD controller.
