Yes. System can boot with modprobe.blacklist=amdgpu & systemd.unit=multi-user.target. This is the way that I tested and got the dmesg. I booted with that configuration and got into command line environment. Than, modprobe efi-pstore which lets the dmesg can be saved into efi storage when system hangs up. Then, modprobe amdgpu manually. If system hangs up at that time, I can get the dmesg with error information in efi storage at next boot.