Parallella Community

The bitstream is what programs the FPGA on boot. The HDMI/FB driver uses the device tree to talk to the virtual HDMI graphics card implemented in the FPGA, You would need a bitstream that has the FPGA support. I have been able to generate my own from the sources posted using Xilinx's Vivado software, but you might be better off just getting a premade one unless you have specific requirements.

Are you running a custom Linux kernel?

The things you need to get FB output are: A HDMI bitstream to program the FPGA, the devicetree files to tell the kernel about the hardware, and a Linux kernel driver for the hardware.