drm/panel: panel-novatak-nt35510: Use backlight power constants

Replace FB_BLANK_ constants with their counterparts from the
backlight subsystem. The values are identical, so there's no
change in functionality or semantics.

Signed-off-by: Thomas Zimmermann <[e-mail]>
Cc: Linus Walleij <[e-mail]>
Cc: Neil Armstrong <[e-mail]>
Cc: Jessica Zhang <[e-mail]>
Reviewed-by: Jessica Zhang <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/panel/panel-novatek-nt35510.c

···
 			bl->props.brightness = nt->conf->wrdisbv;
 		else
 			bl->props.brightness = 255;
-		bl->props.power = FB_BLANK_POWERDOWN;
+		bl->props.power = BACKLIGHT_POWER_OFF;
 		nt->panel.backlight = bl;
 	}