ARM: 6281/1: video/imxfb.c: allow usage without BACKLIGHT_CLASS_DEVICE

From: Eric Bénard <eric@eukrea.com>

This fixes the following build failure:

	drivers/built-in.o: In function `imxfb_probe': mxcmmc.c:(.init.text+0xd04): undefined reference to `backlight_device_register'
	drivers/built-in.o: In function `imxfb_remove': mxcmmc.c:(.devexit.text+0x44): undefined reference to `backlight_device_unregister'

that was introduced by

	7a2bb23c (imxfb: add pwmr controlled backlight support)

Signed-off-by: Eric Bénard <eric@eukrea.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>