video: atmel needs FB_BACKLIGHT

The atmel frame buffer driver doesn't build if FB_BACKLIGHT
is disabled, so select it here as we do for lots of other
drivers.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: linux-fbdev@vger.kernel.org
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>