Added acpi_osi="!Windows2012" to the boot options based on advice in https://wiki.archlinux.org/index.php/Backlight.
Mounting the efi at /boot is simpler but after kernel upgrades I need to manually run grub-mkconfig -o /boot/grub/grub.cfg.
Installed the infinality-bundle.
In /etc/default/grub:
GRUB_CMDLINE_LINUX="add_efi_memmap i915.i915_enable_rc6=1 pcie_aspm=force drm.vblankoffdelay=1 i915.semaphores=1 acpi_osi= video.use_native_backlight=1"