drm/msm/dpu: handle pipes as array

drm/msm/dpu: handle pipes as array

There are 2 pipes in a drm plane at most currently, while 4 pipes are
required for quad-pipe case. Generalize the handling to pipe pair and
ease handling to another pipe pair later. Store pipes in array with
removing dedicated r_pipe.

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/675406/
Link: https://lore.kernel.org/r/20250918-v6-16-rc2-quad-pipe-upstream-4-v16-4-ff6232e3472f@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>