Could you please open a terminal then type in sudo nano /etc/default/grub please?
Once in the editor, use the arrows key on your keyboard to navigate toward the line where it says GRUB_CMDLINE_LINUX_DEFAULT=
Then add acpi_backlight=vendor to the end, but befor the last " (quotation mark)!
Hit CTRL + S to save, then CTRL + X to exit the editor.
GRUB_DEFAULT=saved
GRUB_TIMEOUT=10
GRUB_TIMEOUT_STYLE=hidden
GRUB_DISTRIBUTOR="Manjaro"
GRUB_CMDLINE_LINUX_DEFAULT="quiet apparmor=1 security=apparmor resume=UUID=d2ac6ecc-cb0e-4f82-b617-1792c08b94b3 udev.log_priority=3 acpi_backlight=vendor"
GRUB_CMDLINE_LINUX=""
# If you want to enable the save default function, uncomment the following
# line, and set GRUB_DEFAULT to saved.
GRUB_SAVEDEFAULT=true
# Preload both GPT and MBR modules so that they are not missed
GRUB_PRELOAD_MODULES="part_gpt part_msdos"
# Uncomment to enable booting from LUKS encrypted devices
#GRUB_ENABLE_CRYPTODISK=y
# Uncomment to use basic console
GRUB_TERMINAL_INPUT=console
# Uncomment to disable graphical terminal
#GRUB_TERMINAL_OUTPUT=console
# The resolution used on graphical terminal
# note that you can use only modes which your graphic card supports via VBE
# you can see them in real GRUB with the command 'videoinfo'
GRUB_GFXMODE=auto
# Uncomment to allow the kernel use the same resolution used by grub
GRUB_GFXPAYLOAD_LINUX=keep
# Uncomment if you want GRUB to pass to the Linux kernel the old parameter
# format "root=/dev/xxx" instead of "root=/dev/disk/by-uuid/xxx"
#GRUB_DISABLE_LINUX_UUID=true
# Uncomment to disable generation of recovery mode menu entries
GRUB_DISABLE_RECOVERY=true
# Uncomment and set to the desired menu colors. Used by normal and wallpaper
# modes only. Entries specified as foreground/background.
GRUB_COLOR_NORMAL="light-gray/black"
GRUB_COLOR_HIGHLIGHT="green/black"
# Uncomment one of them for the gfx desired, a image background or a gfxtheme
#GRUB_BACKGROUND="/usr/share/grub/background.png"
GRUB_THEME="/usr/share/grub/themes/manjaro/theme.txt"
# Uncomment to get a beep at GRUB start
#GRUB_INIT_TUNE="480 440 1"
I changed acpi_backlight to native based on this question and the error Failed to start Load/Save #tness of backlight:acpi_video0 vanished.
But I still see amdgpu_bl0