platform/x86: thinkpad_acpi: not loading brightness_init when _BCL invalid

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: Aaron Ma <aaron.ma@canonical.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>