Replace wl_container_of with pepper_container_of

Replace wl_container_of with pepper_container_of

wl_container_of requires a variable to be declared to get the pointer to the
container. We can avoid this by using pepper_container_of by giving a type name
instead of the variable name.

Change-Id: I9afbe84f3177e41e577f0c9d6cad628e9216ceb3