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.orgReviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

drivers/platform/x86/thinkpad_acpi.c

@@ -6798,10 +6798,7 @@ static int brightness_set(unsigned int value)
 
 static int brightness_update_status(struct backlight_device *bd)
 {
-	unsigned int level =
-		(bd->props.fb_blank == FB_BLANK_UNBLANK &&
-		 bd->props.power == FB_BLANK_UNBLANK) ?
-				bd->props.brightness : 0;
+	int level = backlight_get_brightness(bd);
 
 	dbg_printk(TPACPI_DBG_BRGHT,
 			"backlight: attempt to set level to %d\n",