drm/imx: parallel-display: check return code from of_get_drm_display_mode()

of_get_drm_display_mode() may fail. Check its return code and bail out
on error.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>