cxl/pci: Drop idr.h

Commit 3d135db5

 ("cxl/core: Move memdev management to core") left
this straggling include for cxl_memdev setup. Clean it up.

Cc: Ben Widawsky <ben.widawsky@intel.com>
Reported-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Ben Widawsky <ben.widawsky@intel.com>
Link: https://lore.kernel.org/r/163116434668.2460985.12264757586266849616.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>