drm/msm/dpu: introduce struct dpu_sw_pipe

drm/msm/dpu: introduce struct dpu_sw_pipe

Wrap SSPP and multirect index/mode into a single structure that
represents software view on the pipe used.

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/527326/
Link: https://lore.kernel.org/r/20230316161653.4106395-8-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>