ACPI / video: Fix use_native_backlight selection logic

Commit 751109aa ("ACPI / video: Change the default for
video.use_native_backlight to 1") has changed the default for
use_native_backlight from 0 to 1, but instead of changing
use_native_backlight_dmi to true, and leaving use_native_backlight_param at -1,
it has changed use_native_backlight_param to 1.

This causes acpi_video_use_native_backlight() to always think that a value was
specified through the param, making it impossible to add a dmi based quirk
to force 0 now that the default is 1.

This fixes this by restoring the use_native_backlight_param default to -1, and
instead setting the use_native_backlight_dmi default to true.

Fixes: 751109aa

 (ACPI / video: Change the default for video.use_native_backlight to 1)
Cc: 3.16+ <stable@vger.kernel.org> # 3.16+
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>