platform/x86: thinkpad_acpi: Use backlight helper

platform/x86: thinkpad_acpi: Use backlight helper

Instead of retrieving the backlight brightness in struct
backlight_properties manually, and then checking whether the backlight
should be on at all, use backlight_get_brightness() which does all
this and insulates this from future changes.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Cc: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Mark Gross <markgross@kernel.org>
Cc: ibm-acpi-devel@lists.sourceforge.net
Cc: platform-driver-x86@vger.kernel.org
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20220607184635.1127913-5-steve@sk2.org
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>