drm/ttm: fix ttm_kmap_iter.h kernel-doc warnings

There's no proper way to document function pointer members, but at least
silence the warnings.

Cc: Christian Koenig <[e-mail]>
Cc: Huang Rui <[e-mail]>
Acked-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: Christian König <[e-mail]>
Signed-off-by: Jani Nikula <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/e80[e-mail]

include/drm/ttm/ttm_kmap_iter.h

···
  */
 struct ttm_kmap_iter_ops {
 	/**
-	 * kmap_local() - Map a PAGE_SIZE part of the resource using
+	 * @map_local: Map a PAGE_SIZE part of the resource using
 	 * kmap_local semantics.
 	 * @res_iter: Pointer to the struct ttm_kmap_iter representing
 	 * the resource.
···
 	void (*map_local)(struct ttm_kmap_iter *res_iter,
 			  struct iosys_map *dmap, pgoff_t i);
 	/**
-	 * unmap_local() - Unmap a PAGE_SIZE part of the resource previously
+	 * @unmap_local: Unmap a PAGE_SIZE part of the resource previously
 	 * mapped using kmap_local.
 	 * @res_iter: Pointer to the struct ttm_kmap_iter representing
 	 * the resource.