cxl/core/port: Fix / relax decoder target enumeration

If the decoder is not presently active the target_list may not be
accurate. Perform a best effort mapping and assume that it will be fixed
up when the decoder is enabled.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/164317464406.3438644.6609329492458460242.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>