Fixed the resolution doesn't match LCD resolution when using Systeminfo DISPLAY attribute.

BUG=XWALK-1927
I think the mode is just like the resolution which the display supports. <code>flags</code> indicates which resolution we are using. So we don't need to consider other mode's width and height unless we need such kind of info. http://wayland.freedesktop.org/docs/html/protocol-spec-interface-wl_output.html