cxl/region: Factor out code into cxl_region_setup_poison()

cxl/region: Factor out code into cxl_region_setup_poison()

Poison injection setup code is embedded in cxl_region_probe(). For
improved encapsulation, readability, and maintainability, factor out
code into function cxl_region_setup_poison().

This patch is a prerequisite to disable poison by region offset for
Normalized Addressing.

No functional changes.

Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Signed-off-by: Robert Richter <rrichter@amd.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Link: https://patch.msgid.link/20260114164837.1076338-13-rrichter@amd.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>