The root limitation when using the KMS API is deliberate. If your program only
wants to render things then opening/using a render node is what you want.

Looking at the problem from another angle:
As of recently one could use X server without root access, although that
involves using systemd-logind. Not 100% if that will do exactly what you want
but having a quick look at the code and checking David's blog [1] on the topic
won't hurt.

[1] https://dvdhrm.wordpress.com/