drm/msm/dpu: rework dpu_plane_sspp_atomic_update()

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.

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Tested-by: Abhinav Kumar <quic_abhinavk@quicinc.com> # sc7280
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/527337/
Link: https://lore.kernel.org/r/20230316161653.4106395-23-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>