When _BCL invalid, disable thinkpad_acpi backlight brightness control.

brightness_enable is already checked at the beginning.
Most new thinkpads are using GPU driver to control brightness now,
print notice when enabled brightness control even when brightness_enable = 1.
Signed-off-by: NAaron Ma <aaron.ma@canonical.com>
Signed-off-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>