[FIX] models: improve implementation of _compute_display_name()

The method was expecting that name_get() returns complete and in-order values.
Because of this, some records in the recordset could end up without a value.
[FIX] display problem in graph view in some cases

Recently, the graph view was changed to prevent changing groupbys
for active custom filters.  Unfortunately, I did not take into account
the fact that pivot table can be drawn in part when expanding rows.

The "frozen" parameter was then undefined and caused display problems,
this patch should fix the issue.