of: Decrement refcount of previous endpoint in of_graph_get_next_endpoint

Decrementing the reference count of the previous endpoint node allows to
use the of_graph_get_next_endpoint function in a for_each_... style macro.
All current users of this function that pass a non-NULL prev parameter
(that is, soc_camera and imx-drm) are changed to not decrement the passed
prev argument's refcount themselves.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Acked-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>