In practical terms this doesn't make a lot of difference for Plan 9 as it is now. Dev/draw makes use of only a few simple 2d operations for "accelerated" graphics, and the dma engine on the raspberry pi SoC provides a 2d memory-to-memory mode which should be sufficient to do these on the ARM side.
But if anyone is ambitious enough to do something more high level - Plan 9 movie player? - it should be useful to be able to see the API used between the linux high level graphics libraries and the GPU firmware, instead of having to reverse engineer the protocol on the wire. Now we get to reverse engineer the linux library source instead. (In my experience linux software can be so convoluted that this may not be an improvement, but maybe in this case we'll be lucky.)
