cxl/bus: Populate the target list at decoder create

cxl/bus: Populate the target list at decoder create

As found by cxl_test, the implementation populated the target_list for
the single dport exceptional case, it missed populating the target_list
for the typical multi-dport case.

Walk the hosting port's dport list and populate based on the passed in
map.

Move devm_cxl_add_passthrough_decoder() out of line now that it does the
work of generating a target_map.

Before:
$ cat /sys/bus/cxl/devices/root2/decoder*/target_list
0

0

After:
$ cat /sys/bus/cxl/devices/root2/decoder*/target_list
0
0,1,2,3
0
0,1,2,3

Where root2 is a CXL topology root object generated by 'cxl_test'.

Acked-by: Ben Widawsky <ben.widawsky@intel.com>
Link: https://lore.kernel.org/r/162982125942.1124374.13787583357587804107.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>