drm/atomic: remove duplicated assignment of old_plane_state

old_plane_state is already assigned to old_state->plane_states[i] inside
for_each_plane_in_state(). Here we remove an the extra assignment.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>