msm: mdss: Fix debugfs string overwrite

If more than one display paths are active, like split display or external
mirroring, and stat debugfs node is read then debug data for only one
display is shown. Fix this by using correct offsets.

Change-Id: Ia8b36eb3105f6df8f096ff544b82cc49c9b31a48
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>