cxl/mbox: Convert 'enabled_cmds' to DECLARE_BITMAP

Define enabled_cmds as an embedded member of 'struct cxl_mem' rather
than a pointer to another dynamic allocation.

As this leaves only one user of cxl_cmd_count, just open code it and
delete the helper.

Acked-by: Ben Widawsky <ben.widawsky@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/163116436415.2460985.10101824045493194813.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>