mdss: display: last pp_done do the notification at partial update

For left+right partial update case, both pingpong_done are
enabled. The last pingpong_done will do the notification to
signal timeline to release buffer.

Change-Id: Ibfe115d3c9cc05cdc7f9171b9421e85f1cc56157
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>