drm/atomic-helper: fix reference to drm_atomic_helper_commit_planes

The kernel-doc references drm_atomic_commit_planes() which does not
exist. The functions name is drm_atomic_helper_commit_planes().

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161031173646.19453-1-stefan@agner.ch