CRTCs in DSI command mode data path should wait for pingpong done,
instead of vblank, to finish atomic commit.

This change is to enable PP_DONE irq on command mode CRTCs and wait for
this irq happens before atomic commit completion.
Signed-off-by: Hai Li <hali@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>