cxl/region: Fix region HPA ordering validation

cxl/region: Fix region HPA ordering validation

Some regions may not have any address space allocated. Skip them when
validating HPA order otherwise a crash like the following may result:

 devm_cxl_add_region: cxl_acpi cxl_acpi.0: decoder3.4: created region9
 BUG: kernel NULL pointer dereference, address: 0000000000000000
 [..]
 RIP: 0010:store_targetN+0x655/0x1740 [cxl_core]
 [..]
 Call Trace:
  <TASK>
  kernfs_fop_write_iter+0x144/0x200
  vfs_write+0x24a/0x4d0
  ksys_write+0x69/0xf0
  do_syscall_64+0x3a/0x90

store_targetN+0x655/0x1740:
alloc_region_ref at drivers/cxl/core/region.c:676
(inlined by) cxl_port_attach_region at drivers/cxl/core/region.c:850
(inlined by) cxl_region_attach at drivers/cxl/core/region.c:1290
(inlined by) attach_target at drivers/cxl/core/region.c:1410
(inlined by) store_targetN at drivers/cxl/core/region.c:1453

Cc: <stable@vger.kernel.org>
Fixes: 384e624bb211 ("cxl/region: Attach endpoint decoders")
Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/166752182461.947915.497032805239915067.stgit@dwillia2-xfh.jf.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>