staging: video: imx: Select VIDEOMODE_HELPERS for parallel display

Without this, I get the following problem when building kernel:

drivers/built-in.o: In function `imx_pd_connector_get_modes':
/linux-2.6/drivers/staging/imx-drm/parallel-display.c:78: undefined reference to `of_get_drm_display_mode'
make: *** [vmlinux] Error 1

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>