Revert "ACPI / video / i915: No ACPI backlight if firmware expects Windows 8"

We attempted to address a regression introduced by commit a57f7f9
(ACPICA: Add Windows8/Server2012 string for _OSI method.) after which
ACPI video backlight support doesn't work on a number of systems,
because the relevant AML methods in the ACPI tables in their BIOSes
become useless after the BIOS has been told that the OS is compatible
with Windows 8.  That problem is tracked by the bug entry at:

https://bugzilla.kernel.org/show_bug.cgi?id=51231

Commit 8c5bd7a (ACPI / video / i915: No ACPI backlight if firmware
expects Windows 8) introduced for this purpose essentially prevented
the ACPI backlight support from being used if the BIOS had been told
that the OS was compatible with Windows 8 and the i915 driver was
loaded, in which case the backlight would always be handled by i915.
Unfortunately, however, that turned out to cause problems with
backlight to appear on multiple systems with symptoms indicating that
i915 was unable to control the backlight on those systems as
expected.

For this reason, revert commit 8c5bd7a, but leave the function
acpi_video_backlight_quirks() introduced by it, because another
commit on top of it uses that function.

References: https://lkml.org/lkml/2013/7/21/119
References: https://lkml.org/lkml/2013/7/22/261
References: https://lkml.org/lkml/2013/7/23/429
References: https://lkml.org/lkml/2013/7/23/459
References: https://lkml.org/lkml/2013/7/23/81
References: https://lkml.org/lkml/2013/7/24/27
Reported-and-tested-by: James Hogan <james@albanarts.com>
Reported-and-tested-by: Kamal Mostafa <kamal@canonical.com>
Reported-and-tested-by: Jörg Otte <jrg.otte@gmail.com>
Reported-and-tested-by: Steven Newbury <steve@snewbury.org.uk>
Reported-by: Martin Steigerwald <Martin@lichtvoll.de>
Reported-by: Kalle Valo <kvalo@adurom.com>
Tested-by: Joerg Platte <jplatte@naasa.net>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

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