gdb: make dwarf2_find_containing_comp_unit take a dwarf2_per_bfd

While reading another patch, I saw that this function didn't need to
take a dwarf2_per_objfile, but could take a dwarf2_per_bfd instead.
It doesn't change the behavior, but doing this shows that this function
is objfile-independent (can work with only the shared per-bfd data).

Change-Id: I58f9c9cef6688902e95226480285da2d0005d77f