Using a 10.4" 4:3 display with precompiled Yocto 1.6.5

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We try to use a 10.4" 4:3 display to a ConnectCore 6 running Digi Embedded Yocto 1.6.5 (installed the precompiled image).
The Ubuntu Desktop don’t fill the entire screen, but more like a 16:9 format (and then start to paint the desktop again below, a second time.)
Could someone recommend me correct display settings?
Screen is a Mitsubishi AA104XF12-DE2 10.4" XGA, 1024x768. LVDS (6 bit / 8 bit)