drm/atomic: Rename drm_atomic_helper_commit_pre_planes() state argument

The argument contains a pointer to the old state, rename it to old_state
like in all other commit helper functions.

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