cxl/region: Rename function to cxl_port_pick_region_decoder()

cxl/region: Rename function to cxl_port_pick_region_decoder()

Current function cxl_region_find_decoder() is used to find a port's
decoder during region setup. In the region creation path the function
is an allocator to find a free port. In the region assembly path, it
is recalling the decoder that platform firmware picked for validation
purposes.

Rename function to cxl_port_pick_region_decoder() that better
describes its use and update the function's description.

The result of cxl_port_pick_region_decoder() is recorded in a 'struct
cxl_region_ref' in @port for later recall when other endpoints might
also be targets of the picked decoder.

Signed-off-by: Robert Richter <rrichter@amd.com>
Reviewed-by: Gregory Price <gourry@gourry.net>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: "Fabio M. De Francesco" <fabio.m.de.francesco@linux.intel.com>
Tested-by: Gregory Price <gourry@gourry.net>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Link: https://patch.msgid.link/20250509150700.2817697-6-rrichter@amd.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>