mdss: display: wait for ping pong done before update registers

Ping pong done interrupt indicates previous mdp kickoff has been
completed and it is ready for next kickoff. Non-double buffered
registers updated can only be executed after ping pong done to avoid
unexpected side effects when updating hardware while mdp still active.

CRs-fixed: 481408
Change-Id: Ia06ad8dc7663e31132fe066095a3911aecb5d3e4
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>