On phoronix they say some improvement are possible with next kernel versions
https://www.phoronix.com/scan.php?page=news_item&px=AMDGPU-Linux-4.17-Round-1

I've recompiled amdgpu module with this kind of change
https://www.phoronix.com/forums/forum/linux-graphics-x-org-drivers/amd-linux/918649-underclocking-undervolting-the-rx-470-with-amdgpu-pro-success
but in fact, I see that the numbers are already changed because of the bios power state, but that it have no effect. I could not continue investigation.

As far as I understand if one understand which memory have to be read/set to control the i2c, and understand the vbios tables (as read by AtomBiosReader), one could do what amdgpu do when interpreting the vbios pseudocode (x86 if I understand well). all normally is in the amdgpu driver, and as I suspect it could even be done with /dev/mem.
best of course would be to use sysfs peudo files calling subroutins in amdgpu, like it is already proposed for pp_*