cogl/buffer: Don't store "vfuncs" in the struct
This was done during the move to a GObjectified Buffer instance, but in the future when we will move the DriverVtable to an abstract class, this would annoy us.
Given that those functions are only used internally in three instances, just replace them with the actual logic of checking whether tu use malloc fallback path or not.
Helps !4132 (merged)