Remove reference to dwarf2_per_cu_data::dwarf2_per_objfile in queue_and_load_all_dwo_tus

In this context, we know that per_cu->cu will be set, as there is this
assertion:

    gdb_assert (per_cu->cu != NULL)

So in order to remove the dwarf2_per_cu_data::dwarf2_per_objfile
reference in queue_and_load_all_dwo_tus, we can go through per_cu->cu.
This adds a reference to dwarf2_per_cu_data::cu, but it will get removed
eventually, in a subsequent patch.

gdb/ChangeLog:

	* dwarf2/read.c (queue_and_load_all_dwo_tus): Access per_objfile
	data through per_cu->cu.

Change-Id: Id4662828ac3c5bc93fe221df3c9bd9a36a8427ad