[FIX] purchase: locked purchase orders are billable

Before this commit, when the PO was locked:
- the button create bill did not appear on the form view
- on the invoice form view, the field autocomplete did not mention the PO

After this commit, the button create bill is present, and the PO appears in
the auto-complete field of the Vendor bill

OPW 1970537

closes odoo/odoo#32794

Signed-off-by: Lucas Perais (lpe) <lpe@odoo.com>