drm/msm/dpu: split PIPES_PER_STAGE definition per plane and mixer

drm/msm/dpu: split PIPES_PER_STAGE definition per plane and mixer

The stage contains configuration for a mixer pair. Currently the plane
supports just one stage and 2 pipes. Quad-pipe support will require
handling 2 stages and 4 pipes at the same time. In preparation for that
add a separate define, PIPES_PER_PLANE, to denote number of pipes that
can be used by the plane.

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/675408/
Link: https://lore.kernel.org/r/20250918-v6-16-rc2-quad-pipe-upstream-4-v16-5-ff6232e3472f@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>