fbdev: atmel_lcdfb: 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 <tzimmermann@suse.de>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Helge Deller <deller@gmx.de>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Claudiu Beznea <claudiu.beznea@tuxon.dev>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240731123502.1145082-2-tzimmermann@suse.de