drm/msm/dpu: rework dpu_plane_sspp_atomic_update()

Split pipe-dependent code from dpu_plane_sspp_atomic_update() into the
separate function dpu_plane_sspp_update_pipe(). This is one of
preparational steps to add r_pipe support.

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