vout: use designated initializers for display ops
Benefits of not using designated initializers:
- we can't forget a callback
- it's the same in C and C++ (<20)
Benefits of using designated initializers:
- the order of callbacks may not be wrong
- we can read immediately which function is assigned to which callback (especially if some are NULL)
- an optional callback may be added without modifying all the modules
- we can grep a callback name to find its assignments easily
- it is consistent with others xxx_operations in the codebase