drm/atomic-helper: Make zpos property kerneldoc less misleading

kerneldoc for drm_plane_create_zpos_property() says that the DRM core
will automatically calculate the normalized zpos values, but it doesn't
actually do that anymore since commit 38d868e4 ("drm: Don't force
all planes to be added to the state due to zpos"). Instead, drivers are
supposed to explicitly call the drm_atomic_normalize_zpos() function.
Change the kerneldoc comment to reflect that.

v2: reference the commit that introduced the kerneldoc unclarity

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171120150959.26913-1-thierry.reding@gmail.com