drm/msm/dpu: blend pipes per mixer pairs config

drm/msm/dpu: blend pipes per mixer pairs config

Currently, only 2 pipes are used at most for a plane. A stage structure
describes the configuration for a mixer pair. So only one stage is needed
for current usage cases. The quad-pipe case will be added in future and 2
stages are used in the case. So extend the stage to an array with array
size STAGES_PER_PLANE and blend pipes per mixer pair with configuration in
the stage structure.

Signed-off-by: Jun Nie <jun.nie@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/675412/
Link: https://lore.kernel.org/r/20250918-v6-16-rc2-quad-pipe-upstream-4-v16-7-ff6232e3472f@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>