kconfig: gconf: avoid hardcoding model2 in on_treeview2_cursor_changed()

[ Upstream commit cae9cdbcd9af044810bcceeb43a87accca47c71d ]

The on_treeview2_cursor_changed() handler is connected to both the left
and right tree views, but it hardcodes model2 (the GtkTreeModel of the
right tree view). This is incorrect. Get the associated model from the
view.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>