elm_panel: use widget data for getting geometry

elm_panel: use widget data for getting geometry

_elm_panel_evas_object_smart_resize() ends up calling
_elm_panel_elm_layout_sizing_eval() to adjust the layout
according to the updated width and height.
however, evas_object_geometry_get() doesn't return the updated values.
in fact, it is not necessary to call any API since the values are
stored as widget data in _elm_widget_evas_object_smart_resize().

Change-Id: I68467ec07700ee4dea99d36ee10c433608d5f555