drm/msm/sde: add serialize pp support for cmd mode

Frame N waits for pingpong done (frame transfer complete)
before starting frame N+1 transfer on command mode
interface. It is useful to wait for this frame transfer
after Frame N kickoff itself for debugging purpose. This
patch adds the serialize pingpong wait support on
command mode interface.

Change-Id: Ie23311780fb5e8f8cc32f5ccdd47682c9d36c3ab
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>