drm/msm/dpu: support SSPP assignment for quad-pipe case

drm/msm/dpu: support SSPP assignment for quad-pipe case

Currently, SSPPs are assigned to a maximum of two pipes. However,
quad-pipe usage scenarios require four pipes and involve configuring
two stages. In quad-pipe case, the first two pipes share a set of
mixer configurations and enable multi-rect mode when certain
conditions are met. The same applies to the subsequent two pipes.

Assign SSPPs to the pipes in each stage using a unified method and
to loop the stages accordingly.

Signed-off-by: Jun Nie <jun.nie@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Reviewed-by: Jessica Zhang <jessica.zhang@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/675414/
Link: https://lore.kernel.org/r/20250918-v6-16-rc2-quad-pipe-upstream-4-v16-8-ff6232e3472f@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>