drm/msm/dpu: simplify peer LM handling

For each LM there is at max 1 peer LM. Thus there is no need to have a
mask for the peer LM enumeration. Change that to have an ID of the peer LM.

Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Tested-by: Marijn Suijten <marijn.suijten@somainline.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/545352/
Link: https://lore.kernel.org/r/20230704022136.130522-4-dmitry.baryshkov@linaro.org