I don't know if it's helpful, but I'm struggling with a similar issue on my
RPi 3 B+, where the official touchscreen, as well as, the HDMI don't output
any graphics, although detected correctly. (See:
https://lists.yoctoproject.org/pipermail/yocto/2019-May/045277.html)
After experimenting with sumo
Hi!
I'm facing some trouble with the X-Server on the Raspberry Pi 3 B+. No
matter if I use a screen on the HDMI port or running the 7" touch screen,
they remain black. After testing with sumo - where at least a splash screen
was shown - I moved on to thud, as recommended by Andrei Gherzan to sort
