Remove dwarf2_cu->per_cu->dwarf2_per_objfile references

Remove dwarf2_cu->per_cu->dwarf2_per_objfile references

Change spots that access the dwarf2_per_objfile object through this
pattern:

  dwarf2_cu->per_cu->dwarf2_per_objfile

to

  dwarf2_cu->per_objfile

This allows removing many references to
dwarf2_per_cu_data::dwarf2_per_objfile.

Again, I hope the following ChangeLog entry will be fine.  I'd rather not
list all the affected functions, as it would be time-consuming and a bit
pointless.

gdb/ChangeLog:

	* dwarf2/read.c: Replace
	dwarf2_cu->per_cu->dwarf2_per_objfile references with
	dwarf2_cu->per_objfile throughout.

Change-Id: I00f44e88295f70ae805a4b18e8144ca92154612e