Since you are using CM3, is it safe to assume you are using a screen which relies on a 32-bit drm driver? That might just be your issue, as v34 on the RPi3 has upgraded to 64-bit. ie. 32-bit drivers will not work on 64-bit image.
If you need to keep using older drivers, you should stick with the older image (at least until the newer 32-bit image gets released, which depending on demands may just be a thing which happens in the future).
