PCI: add acpi_find_root_bridge_handle

Consolidate finding of a root bridge and getting its handle to the one
inline function. It's cut & pasted on multiple places. Use this new
inline in those.

Cc: kristen.c.accardi@intel.com
Acked-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>