The display controller on ARM SoCs is typically separate from the Mali or PowerVR GPU and is often documented. If there are multiple ARM cores on the SoC then the display may still feel fast enough for basic stuff even though the CPU is doing all the work. Another factor is that some modern GPUs are 3D only, the graphics stack gets a lot more complicated if it has to translate 2D requests from an application into OpenGL calls to the GPU.

NetBSD has a framebuffer driver for TI OMAP CPUs so it could be possible to port it to OpenBSD.