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 <christian.koenig@amd.com>
Cc: Huang Rui <ray.huang@amd.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e80e74ac4b6c5f1df3bc2dd98651ba289aae8e83.1709898638.git.jani.nikula@intel.com