[FIX] purchase: Adding a purchase order on a vendor bill

To keep the same behavior as in 11.0, you can only add a purchase order to a vendor
bill when the vendor bill is in draft. Afterwards it is not longer possible.

opw:803068

addons/purchase/views/account_invoice_views.xml

@@ -59,7 +59,7 @@
         <field name="inherit_id" ref="account.invoice_supplier_form"/>
         <field name="arch" type="xml">
             <field name="reference" position="after" >
-                 <field name="purchase_id" attrs="{'invisible': [('state', '=', 'purchase')]}" class="oe_edit_only" 
+                 <field name="purchase_id" attrs="{'readonly': [('state','not in',['draft'])], 'invisible': [('state', '=', 'purchase')]}" class="oe_edit_only"
                     options="{'no_create': True}"/>
             </field>
             <xpath expr="//field[@name='invoice_line_ids']/tree/field[@name='company_id']" position="after">