https://bugs.freedesktop.org/show_bug.cgi?id=106228
--- Comment #7 from jian-hong@endlessm.com --- Applied both "[PATCH] drm/amdgpu: Read back max backlight value at boot" and "[PATCH] drm/amd/display: Implement backlight_ops.get_brightness" to Linux 4.18.0-rc6 kernel.
I got the same behavior as previous comment.