The Open-Q 2100 display resolution supports up to 720p at 60fps. No, you do not need to connect a FPGA in between the display and CPU to achieve 640 x 400 (60fps). You can control the output resolution through software. To do this, you will need to modify the dts file that contains resolution and timing parameters.
If you need support on modifying the dts file, please contact firstname.lastname@example.org and we can discuss how we can help you make these changes.