I think the answer you got in your forum post is correct: if X.org doesn't have a chipset-specific driver, it's going to fall back to the generic VESA driver, which may not support all of the functions of that graphics chip.
I looked in the manual for that motherboard, and it is very sketchy on graphics issues. It claims "up to 1920x1200" but doesn't justify that with a video memory spec. Given the nature of the Atom and PowerVR, I'd bet it's taking a slice of system RAM for a video buffer, but I don't see anything in the UEFI section of the manual on configuring its size.
You could try forcing X to recognize the video RAM. Say
sudo nano /usr/share/X11/xorg.conf.d/99-vesahack and put this in the newly created file:
This tells X that there is 9 MB of video RAM, enough for 1920x1200 at 4 bytes per pixel. If X won't restart when you do that, try changing it to 6750 kB, for 3 Bpp. I don't hold out much hope that this will work, but it's a thing to try. If it doesn't help, remove
Your best option may be to install a graphics card known to have good X.org support.