msm: mdss: clear previous DSI_VIDEO_DONE interrupt

For DSI DMA command transfer in video mode panels, we need to
wait for DSI video done interrupt to make sure that the commands
are queued at the beginning of next active region. Make sure that
the previous DSI_VIDEO_DONE is cleared before waiting for the next
VIDEO_DONE interrupt. Otherwise, the h/w triggers a false interrupt
for the previous DSI_VIDEO_DONE interrupt immediately after the
DSI_VIDEO_DONE_MASK is enabled.

Change-Id: Ie024e44dff89caf51164490a59b6613d5e4e01ea
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>