SRU update for acpi backlight problems

[Stefan Bader]

https://bugs.launchpad.net/ubuntu/+source/linux/+bug/311716

SRU justification:

Impact: Some laptops had issues with the current backlight control. This is 
because up to then acpi and vendor specific control methods were able/allowed 
to access the video hardware (https://bugs.launchpad.net/bugs/257827). The fix 
for this (taken from upstream) fixes this but causes regressions for others. So 
for stable it should probably be taken back but this would cause an ABI bump.

Fix: I changed the detection code in a (slightly ugly to keep diff minimal) way 
to make the old behaviour the default (generic _and_ vendor control at the same 
time) but leave the infrastructure (no ABI bump) which gives the additional 
ability for those that have problems with that to force either generic (video) 
or vendor specific (vendor) by using the module option acpi_backlight.

Testcase: Boot the kernel and try to adjust backlight levels with FN-keys 
and/or the gnome backlight applet.

Note: going over the original acpi patches, it looks like some vendor drivers 
might have been fixed wrong (eg. sony-laptop.c which bails out on 
!acpi_backlight_suppoert(). But that is the case when acpi is _not_ active).

-- 

When all other means of communication fail, try words!

) which resulted in regression for some others (initial report here and in the 
other
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