cxl: Add cxl_num_decoders_committed() usage to cxl_test

Commit 458ba8189cb4 ("cxl: Add cxl_decoders_committed() helper") missed the
conversion for cxl_test. Add usage of cxl_num_decoders_committed() to
replace the open coding.

Suggested-by: Alison Schofield <alison.schofield@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Fan Ni <fan.ni@samsung.com>
Link: https://lore.kernel.org/r/169929160525.824083.11813222229025394254.stgit@djiang5-mobl3
Signed-off-by: Dan Williams <dan.j.williams@intel.com>