platform/x86: apple-gmux: 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: Hans de Goede <hdegoede@redhat.com>
Cc: Mark Gross <markgross@kernel.org>
Cc: platform-driver-x86@vger.kernel.org
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20220607184635.1127913-3-steve@sk2.orgReviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

drivers/platform/x86/apple-gmux.c

@@ -291,10 +291,7 @@ static int gmux_get_brightness(struct backlight_device *bd)
 static int gmux_update_status(struct backlight_device *bd)
 {
 	struct apple_gmux_data *gmux_data = bl_get_data(bd);
-	u32 brightness = bd->props.brightness;
-
-	if (bd->props.state & BL_CORE_SUSPENDED)
-		return 0;
+	u32 brightness = backlight_get_brightness(bd);
 
 	gmux_write32(gmux_data, GMUX_PORT_BRIGHTNESS, brightness);