Same problem at Ubuntu 20.04 LTS with Radeon RX550. 

Processing triggers for initramfs-tools (0.136ubuntu4) ...
update-initramfs: Generating /boot/initrd.img-5.4.0-21-generic
W: Possible missing firmware /lib/firmware/amdgpu/navi12_gpu_info.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/arcturus_gpu_info.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/arcturus_asd.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/arcturus_sos.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/navi12_asd.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/navi12_sos.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/vega20_ta.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/arcturus_rlc.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/arcturus_mec2.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/arcturus_mec.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/navi12_rlc.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/navi12_mec2.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/navi12_mec.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/navi12_me.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/navi12_pfp.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/navi12_ce.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/arcturus_sdma.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/navi12_sdma1.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/navi12_sdma.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/navi10_mes.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/navi12_vcn.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/arcturus_vcn.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/navi12_smc.bin for module amdgpu
W: Possible missing firmware /lib/firmware/amdgpu/arcturus_smc.bin for module amdgpu
I: The initramfs will attempt to resume from /dev/nvme0n1p2
I: (UUID=baebf59d-af03-4914-806e-c2672a0210d8)
I: Set the RESUME variable to override this.
Processing triggers for sgml-base (1.29.1) ...
Refreshing boot menu
INFO Updating GRUB menu                           

Other info:

martin@slazer:~$ lspci -k | grep -EA3 'VGA|3D|Display'
0a:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Lexa PRO [Radeon 540/540X/550/550X / RX 540X/550/550X] (rev c7)
	Subsystem: Sapphire Technology Limited Lexa PRO [Radeon RX 550]
	Kernel driver in use: amdgpu
	Kernel modules: amdgpu
martin@slazer:~$ lsmod | grep "kms\|drm"
drm_kms_helper        184320  1 amdgpu
syscopyarea            16384  1 drm_kms_helper
sysfillrect            16384  1 drm_kms_helper
sysimgblt              16384  1 drm_kms_helper
fb_sys_fops            16384  1 drm_kms_helper
drm                   491520  12 gpu_sched,drm_kms_helper,amdgpu,ttm
martin@slazer:~$ find /dev -group video
/dev/fb0
/dev/dri/card0
martin@slazer:~$ cat /proc/cmdline
BOOT_IMAGE=/BOOT/ubuntu_9dd6hp@/vmlinuz-5.4.0-21-generic root=ZFS=rpool/ROOT/ubuntu_9dd6hp ro quiet splash vt.handoff=1
martin@slazer:~$ find /etc/modprobe.d/
/etc/modprobe.d/
/etc/modprobe.d/alsa-base.conf
/etc/modprobe.d/iwlwifi.conf
/etc/modprobe.d/blacklist-rare-network.conf
/etc/modprobe.d/blacklist-ath_pci.conf
/etc/modprobe.d/blacklist.conf
/etc/modprobe.d/blacklist-modem.conf
/etc/modprobe.d/amd64-microcode-blacklist.conf
/etc/modprobe.d/blacklist-oss.conf
/etc/modprobe.d/blacklist-firewire.conf
/etc/modprobe.d/blacklist-framebuffer.conf
/etc/modprobe.d/intel-microcode-blacklist.conf