This Bug #801348 looks pretty similar to Bug #651741, which I reported originally in 2011. The hardware is different though.
You can probably make it work by putting this in /etc/X11/xorg.conf:
Section "Device"
Option "Backlight" "intel_backlight"
Identifier "Card0"
Driver "intel"
BusID "PCI:0:2:0"
EndSection
This requires xserver-xorg-video-intel 2.20.7 or later;
you have 2:2.21.15-2+b2, which is new enough.
Linux apparently has a blacklist for buggy ACPI backlight
interfaces nowadays:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/acpi/video.c?id=refs/tags/v3.16#n418
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/acpi/acpi_video.c?id=refs/tags/v4.7#n409
I should finally file a bug in the kernel Bugzilla and ask them
to add my Sony Vaio VPCYA1V9E to the blacklist.
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