Running Android on a mainline graphics stack

Your standards for "really terrible" might be excessively high :-) . I could be misremembering but I think some low-cost Broadcom chips (maybe BCM21664 or similar?), used in e.g. some Samsung phones a few years ago, had VideoCore 4 but with all the fun bits stripped out to minimise cost. That included removing the VPU and any clever display hardware, so compositing had to be done entirely in OpenGL. Admittedly that's a fairly old chip, I don't know if any more recent ones have similar limitations.

(Proper non-stripped-down VC4, like in Raspberry Pi, does compositing with HVS <https://dri.freedesktop.org/docs/drm/gpu/vc4.html>. The mainline vc4 driver uses that to implement DRM atomic mode setting.)