mdss: mdp: wait for pingpong done for second control

In the current implementation, we are waiting for ping pong
interrupt only for the first control. There is a possibility
that the second control takes more time than the first. As the
commit call is asynchronous next kickoff could get triggered.
This causes dsi fifo to underflow and eventually pingpong timeout
are observed.

This change fixes the issue by waiting for second control's
pingpong done interrupt, to ensure that the tx to dsi
controller is complete before the next commit is triggered.

Change-Id: Ie8caf36744b3f5860962ad30d19d61d0a6de2763
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>