backlight: corgi-lcd: 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.

Only change the driver's backlight device, but leave the LCD
device as-is.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20240624152033.25016-5-tzimmermann@suse.de


Signed-off-by: Lee Jones <lee@kernel.org>