msm: mdss: fix single layer vote for pipe staged at higher levels

BW calculation for single layer requires iterative search on how many
pipes are staged on a layer mixer. In the current implementation,
starting source split, container used to store staged pipes has two
entries per stage. Based on this maximum entries to search for single
layer has also increased. Update single layer iterative search based on
this new changes.

Change-Id: I2e65567fd795d71ca5ded0fa56c602267fe46768
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>