cxl/core: Drop is_cxl_decoder()

This helper was only used to identify the object type for lockdep
purposes. Now that lockdep support is done with explicit lock classes,
this helper can be dropped.

Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Adam Manzanares <a.manzanares@samsung.com>
Link: https://lore.kernel.org/r/165603874340.551046.15491766127759244728.stgit@dwillia2-xfh


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