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.