drm/xe/uapi: add some kernel-doc for region query

Since we need to extend this, we should also take the time to add some
basic kernel-doc here for the existing bits. Note that this is all still
subject to change when upstreaming.

Also convert XE_MEM_REGION_CLASS_* into an enum, so we can more easily
create links to it from other parts of the uapi.

Suggested-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Filip Hazubski <filip.hazubski@intel.com>
Cc: Carl Zhang <carl.zhang@intel.com>
Cc: Effie Yu <effie.yu@intel.com>
Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>