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