ACPI / video: Quirk initial backlight level 0

commit 2c62333 upstream.

Some firmware doesn't initialize initial backlight level to a proper
value and _BQC will return 0 on first time evaluation. We used to be
able to detect such incorrect value with our code logic, as value 0
normally isn't a valid value in _BCL. But with the introduction of Win8,
firmware begins to fill _BCL with values from 0 to 100, now 0 becomes
a valid value but that value will make user's screen black. This patch
test initial _BQC for value 0, if such a value is returned, do not use
it.

References: https://bugzilla.kernel.org/show_bug.cgi?id=64031
References: https://bugzilla.kernel.org/show_bug.cgi?id=61231
References: https://bugzilla.kernel.org/show_bug.cgi?id=63111
Reported-by: Qingshuai Tian <qingshuai.tian@intel.com>
Tested-by: Aaron Lu <aaron.lu@intel.com> # on "Idealpad u330p"
Reported-and-tested-by: <erno@iki.fi> # on "Acer Aspire V5-573G"
Reported-and-tested-by: Kirill Tkhai <tkhai@yandex.ru> # on "HP 250 G1"
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/acpi/video.c

@@ -848,7 +848,7 @@ acpi_video_init_brightness(struct acpi_video_device *device)
 		for (i = 2; i < br->count; i++)
 			if (level_old == br->levels[i])
 				break;
-		if (i == br->count)
+		if (i == br->count || !level)
 			level = max_level;
 	}