I have an update about that.
libffi7 was requested by libwayland-server-0.20.0 that was in /opt/amdgpu/lib but there was an error with this file because it should not be here, probably an old installation.
I found that with 'dpkg -S libwayland-server' and no one of amdgpu packages for ubuntu provide this file.
So I just remove lib directory under /opt/amdgpu and reinstalled all the stuff, and it works.
My system is a Debian Sid, so actually the latest version of AMD driver works.