drm/msm/dsi-staging: add support for dsi null packet insertion

For DSI6G v1.4.0 and above, DSI controller has support to insert
NULL packets in the middle of pixel data or DMA packets to avoid
HS-LP-HS transitions while the transfer is taking place.

Change-Id: I780495feca2030705f3f89909700a8a9d571564c
Signed-off-by: Sravanthi Kollukuduru <skolluku@codeaurora.org>