run lxde on debootstrap maked UBUNTU

Hi all! I make a ubuntu raring rootfs by deboootstrap, and boot the beagle bone black correctly.
After apt-get installing the lxde. I can see the lxdm is runing by ps -aux.
but nothing was show on the hdmi connect to LCD.
fbset show that the fb0 is set to 1024x768,but timing is all 0.
what should I do to make the X correctly and show to hdmi port?


Setup an xorg.conf overide