LVDS black screen if external VGA plugged at power on. https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1393945
The gma500 is basically undocumented. Although its on Intel hardware it's mostly an imagination device with no manuals. The driver was built out of the pieces that were used with a proprietary user server without documentation. If you boot with all the drm debug turned on then it may give us some clues, but probably we can't fix it.