Remove dwarf2_per_cu_data::dwarf2_per_objfile reference in cutu_reader::keep

Here, it should be safe to use dwarf2_per_cu_data->cu->per_objfile, as
we know that dwarf2_per_cu_data->cu will be set at this point.

Note that this adds a reference to dwarf2_per_cu_data::cu, which we'll
want to remove later, but the current focus is to remove references to
dwarf2_per_cu_data::dwarf2_per_objfile.  We'll deal with that in a
subsequent patch.

gdb/ChangeLog:

	* dwarf2/read.c (cutu_reader::keep): Access dwarf2_per_objfile
	object through m_this_cu->cu.

Change-Id: I8dc26d4db021e0b9e9306eb033965b2704bba87c