gdb/dwarf: change some per-objfile functions to be per-bfd

I identified the following functions that currently take a
dwarf2_per_objfile, but could take a less specific dwarf2_per_bfd.

 - try_open_dwop_file
 - open_dwo_file
 - open_dwp_file

The uses of the per-objfile object in try_open_dwop_file can be replaced
with equivalent per-bfd ones.

Change-Id: Ia31fa0b988375e86a715ee863d4ec3c572ce89c0
Approved-By: Tom Tromey <tom@tromey.com>