drm/xe: Add documentation for mem_type

mem_type field was added in commit d8b52a02 ("drm/xe: Implement
stolen memory.") to designate the TTM memory type for that mgr. Add
kernel-doc with its description.
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/xe/xe_ttm_vram_mgr_types.h

@@ -27,7 +27,7 @@ struct xe_ttm_vram_mgr {
 	u64 default_page_size;
 	/** @lock: protects allocations of VRAM */
 	struct mutex lock;
-
+	/** @mem_type: The TTM memory type */
 	u32 mem_type;
 };