drm/omap: Don't dereference list head when the connectors list is empty

The connectors list iterator returns the list head when the list is
empty. Fix it by returning NULL in that case.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Rob Clark <robdclark@gmail.com>