OMAPDSS: fix dss_init_ports error handling

The return value of dss_init_ports() is not handled at all, causing
crashes later if the call failed.

This patch adds the error handling, and we also move the call to a
slightly earlier place to make bailing out easier.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>