cxl/region: Move cxl_dpa_to_region() work to the region driver

cxl/region: Move cxl_dpa_to_region() work to the region driver

[ Upstream commit b98d042698a32518c93e47730e9ad86b387a9c21 ]

This helper belongs in the region driver as it is only useful
with CONFIG_CXL_REGION. Add a stub in core.h for when the region
driver is not built.

Signed-off-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/05e30f788d62b3dd398aff2d2ea50a6aaa7c3313.1714496730.git.alison.schofield@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Stable-dep-of: 285f2a088414 ("cxl/region: Avoid null pointer dereference in region lookup")
Signed-off-by: Sasha Levin <sashal@kernel.org>