msm: mdss: fix incorrect stage & un-stage when using source split

With source-split feature, a pipe can be staged across two layer
mixers (LM) and/or two pipes can be staged on a LM same stage. Current
implementation stages source split pipe into incorrect container index
for right LM. Similarly while un-staging it calculates incorrect index
if more than two pipes are staged on a single LM. Fix this to resolve
MDP pipeline hangs.

Change-Id: I89e0f5946cb330dc918641486728baea856de574
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>