drm/atomic-helpers: Fix documentation typos and wrong copy&paste

The kerneldoc blocks for the drm_atomic_helper_*_set_property()
functions seem to have been copied from the plane disable handler
without being properly updated. Fix them.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>