cxl/mbox: Move build of user mailbox cmd to a helper functions

In preparation for moving the construction of a mailbox command
to the validation path, extract the work into a helper functions.

Signed-off-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Link: https://lore.kernel.org/r/493d7618a846d787c3ae28778935ca35e2b85eed.1648687552.git.alison.schofield@intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>