turnip: move enum translation functions to a common header

Instead of having these functions sprinkled around the driver (and ending with a duplicated tu6_compare_func for example), move everything to a common header (using the previously unused tu_util.h).

Also applied some simplifications: using a cast when the HW enum matches the VK enum, and using a lookup table when it makes sense (which is IMO nicer than the switch case way).