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>
Link: https://lore.kernel.org/r/20241213100647.200598-1-tzimmermann@suse.de


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