drm/msm/dpu: move pipe_hw to dpu_plane_state

In preparation to adding fully virtualized planes, move struct
dpu_hw_sspp instance from struct dpu_plane to struct dpu_plane_state, as
it will become a part of state (allocated during atomic check) rather
than part of a plane (allocated during boot).

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>