elm_panel: use widget data for getting geometry

elm_panel: use widget data for getting geometry

_elm_panel_efl_gfx_size_set() ends up calling
_elm_panel_elm_layout_sizing_eval() to adjust the layout
according to the updated width and height.
however, evas_object_geometr_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_efl_gfx_size_set().