drm/atomic-helper: Rename commmit_post/pre_planes

These names only make sense because of backwards compatability with
the order used by the crtc helper library. There's not really any real
requirement in the ordering here.

So rename them to something more descriptive and update the kerneldoc
a bit. Motivated in a discussion with Laurent about how to restore
plane state for dpms for drivers with runtime pm.

v2: Squash in fixup from Stephen Rothwell to fix a conflict with
tegra.

Cc: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Acked-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>