Chromebook plus uses the RockChip RK3399 cpu, that for graphics uses Mali T860 MP4.
Main effort in developing graphics drivers for Mali was Luc Verhaegen (
http://libv.livejournal.com/), but he lost will to keep developing it. Think it just supported the mali 200 and 400 series. Still may gives some ideas.
Rob Clark (
http://bloggingthemonkey.blogspot.fr/) work on freedreno was quite inspirational, and written in a modern way, so may be a better place to start - but you'll see he now suggests modesetting + glamor, so software solution. Rob is a full on professional btw, he worked in graphics for a living - so he's ahead of the field, even when working on graphics chips he knows nothing about.
I assume you still have chrome os on the machine? I'd dig about in that and see what graphic drivers you can find, may be easiest to use those.
So probably short answer would be, to write a graphics driver you need to be committed, but don't let that stop you ...
Note added later, I think to do modesetting + glamor, that you need a DRM/KMS driver - I'm not sure if that exists, but that would be an easier direction ... I'd also aim to make it a gallium driver, to keep it modern