https://insights.linuxfoundation.org/project/korg/contributo...
It looks like they're second to Intel, at least by LF's metric. That said driver code tends to be take up a lot of space compared to other areas. Just look at the mass of AMD template garbage here: https://github.com/torvalds/linux/tree/master/drivers/gpu/dr...
Intel has long been a big contributor--mostly driver stuff as I understand it. (Intel does a lot more software work than most people realize.) Samsung was pretty high on the list at one point as well. My grad school roommate (now mostly retired though he keeps his hand in) was in the top 10 individual list at one point--mostly for networking-related stuff.