deposit: change to use update_field() in fillForm

This works after as well as before merging these pull reuests:

https://github.com/inveniosoftware/invenio/pull/1893
https://github.com/inveniosoftware/invenio/pull/1964

It makes this commit unneeded:
https://github.com/jmartinm/invenio/commit/d7d8dd804c7911afc23501fa700d0ac60f543377