Hi Ekta,
If you want to just make it display field you can use get i method itself. Mostly get a method will be used to hide/display the fields from configuration.
In general you can check whether view is in display mode or edit mode based on that you can make field as display field by setting rv_disable = TRUE in get i method.
I hope for your requirement you can stright forwards make the field disables once value has been entered in the field.
You need to check the object id field to identify whether the edit is first time or after saving, because object id will be generated for the document after save only.
If object id is available and your attribute is not initial make the rv_disable = TRUE in get i method.
Best Regards,
Dharmakasi.