dax/hmem: Factor HMEM registration into __hmem_register_device()

dax/hmem: Factor HMEM registration into __hmem_register_device()

Separate the CXL overlap check from the HMEM registration path and keep
the platform-device setup in a dedicated __hmem_register_device().

This makes hmem_register_device() the policy entry point for deciding
whether a range should be deferred to CXL, while __hmem_register_device()
handles the HMEM registration flow.

No functional changes.

Signed-off-by: Smita Koralahalli <Smita.KoralahalliChannabasappa@amd.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://patch.msgid.link/20260322195343.206900-3-Smita.KoralahalliChannabasappa@amd.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>