cxl/decoder: Move decoder register programming to a helper

cxl/decoder: Move decoder register programming to a helper

In preparation for converting to rw_semaphore_acquire semantics move the
contents of an open-coded {down,up}_read(&cxl_dpa_rwsem) section to a
helper function.

Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Jonathan Cameron <jonathan.cameron@huawei.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Alison Schofield <alison.schofield@intel.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Link: https://patch.msgid.link/20250711234932.671292-4-dan.j.williams@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>