Apply "[PATCH] drm/amdgpu: Read back max backlight value at boot" to Linux
4.18.0-rc6 kernel.

I disable all of the brightness setting applications, including masking
systemd-backlight service and than reboot.
"cat /sys/class/backlight/amdgpu_bl0/brightness" before set brightness manually
again and it reads back 255 (the maximum brightness).

If I unmask systemd-backlight service and than reboot for test, "cat
/sys/class/backlight/amdgpu_bl0/brightness" before set brightness manually
again and it reads back the brightness which is saved before last reboot.