ACPI / i915: Update the condition to ignore firmware backlight change request

Some of the Thinkpads' firmware will issue a backlight change request
through i915 operation region unconditionally on AC plug/unplug, the
backlight level used is arbitrary and thus should be ignored. This is
handled by commit 0b9f7d93 (ACPI / i915: ignore firmware requests
for backlight change). Then there is a Dell laptop whose vendor backlight
interface also makes use of operation region to change backlight level
and with the above commit, that interface no long works. The condition
used to ignore the backlight change request from firmware is thus
changed to: if the vendor backlight interface is not in use and the ACPI
backlight interface is broken, we ignore the requests; oterwise, we keep
processing them.

Fixes: 0b9f7d93 (ACPI / i915: ignore firmware requests for backlight change)
Link: https://lkml.org/lkml/2014/9/23/854


Reported-and-tested-by: Pali Rohár <pali.rohar@gmail.com>
Cc: 3.16+ <stable@vger.kernel.org> # 3.16+
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>