ACPI / video: ignore BIOS initial backlight value for HP Pavilion g6

This patch addresses kernel bug 56661. BIOS reports an incorrect
backlight value, causing the driver to switch off the backlight
completely during startup. This patch ignores the incorrect value from
BIOS.

References: https://bugzilla.kernel.org/show_bug.cgi?id=56661
Signed-off-by: Ash Willis <ashwillis@programmer.net>
Cc: All <stable@vger.kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>