drm/panel: panel-asus-z00t-tm5p5-n35596: Use backlight helper

backlight_properties.fb_blank is deprecated. The states it represents
are handled by other properties; but instead of accessing those
properties directly, drivers should use the helpers provided by
backlight.h.

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: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220616172316.1355133-2-steve@sk2.org

drivers/gpu/drm/panel/panel-asus-z00t-tm5p5-n35596.c

@@ -215,14 +215,9 @@ static const struct drm_panel_funcs tm5p5_nt35596_panel_funcs = {
 static int tm5p5_nt35596_bl_update_status(struct backlight_device *bl)
 {
 	struct mipi_dsi_device *dsi = bl_get_data(bl);
-	u16 brightness = bl->props.brightness;
+	u16 brightness = backlight_get_brightness(bl);
 	int ret;
 
-	if (bl->props.power != FB_BLANK_UNBLANK ||
-	    bl->props.fb_blank != FB_BLANK_UNBLANK ||
-	    bl->props.state & (BL_CORE_SUSPENDED | BL_CORE_FBBLANK))
-		brightness = 0;
-
 	dsi->mode_flags &= ~MIPI_DSI_MODE_LPM;
 
 	ret = mipi_dsi_dcs_set_display_brightness(dsi, brightness);