platform/x86: dell-uart-backlight: Use blacklight power constant

The backlight subsystem has gotten its own power constants. Replace
FB_BLANK_UNBLANK with BACKLIGHT_POWER_ON.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Hans de Goede <hansg@kernel.org>
Link: https://lore.kernel.org/r/20250715122643.137027-2-tzimmermann@suse.de


Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>