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I updated to the latest kernel, removed linux-git. Everything works with acpi_backlight=vendor.
Thanks you loqs!
For anybody with the same problem, here is the final fix:
1. Go to the /etc/default/grub and find line with GRUB_CMDLINE_LINUX_DEFAULT
2. Add acpi_backlight=vendor to that value
3. Regenerate grub config with grub-mkconfig -o /boot/grub/grub.cfg
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