to know the package relative to amdgpu try
" dpkg -l| grep amdgpu|grep -v ubuntu16"
(the grep -v ubuntu16 is because some adgpu libs are from core ubuntu, not amdgpu-pro. check each name)
but there are also many other pakage related but without the name
then do somethin like
"dpkg --purge amgdpu-core amdgpu-pro-core  amdgpu-dkms ... and so on"
or if you are lazy and feel lucky (check the list generated before)
"dpkg --purge `dpkg -l| grep amdgpu|grep -v ubuntu| awk '{ print $2;}'` "
check if there are pachage with failed dependencies with
"apt-get install"
if there are, purge them too...

and then reinstall (--opencl=legacy and probably --headless unless you use X11 on your GPU).

I have a Sapphire RX580, so it should work...