The kernel doc explained what needs to happen, but not how to most
easily accomplish that using the functions. Fix that.

Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: NBoris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: NNeil Armstrong <narmstrong@baylibre.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170524145212.27837-14-daniel.vetter@ffwll.ch