drm/msm/dsi-staging: trigger dma command during video active region

For dsi video mode use case when engine is transmitting pixel data
then as per recommendation wait for video frame done interrupt and
then ensure dsi engine is in active region to queue the dma command.

Change-Id: I19bc38eb149ad3ea92d6659b8684fc7d202427ac
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>