Turns out we didn't even need to modify the kernel for this. By choosing simplefb and the 'tiny' DRM drivers for initrd, we get simpledrm.ko as it existed in our older kernels...
https://code.launchpad.net/~vanvugt/ubuntu/+source/initramfs-tools/+git/initramfs-tools/+merge/462482
Turns out we didn't even need to modify the kernel for this. By choosing simplefb and the 'tiny' DRM drivers for initrd, we get simpledrm.ko as it existed in our older kernels...
https:/ /code.launchpad .net/~vanvugt/ ubuntu/ +source/ initramfs- tools/+ git/initramfs- tools/+ merge/462482