drm: shmobile: Add dependency on BACKLIGHT_CLASS_DEVICE

The driver registers a backlight device and thus requires
BACKLIGHT_CLASS_DEVICE to be selected to avoid compilation breakages.

Cc: stable@vger.kernel.org
Reported-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>