How are you writing into the /dev/graphcis/fb0?
Are you using the right IOCTLs? or are you trying to cat a buffer into it. ‘cat’ -ing a buffer will not work.
As you have already downloaded the source, you can refer to bootable\recovery\minui\graphics_fbdev.c for the IOCTLs to be used.
Also try disabling the Display Hardware using its overlay hardware to composite the UI by ‘Turning ON’ the following: (Settings -> Developer Options -> Disable Hardware Overlays).