msm: mdss: add support for DSI NULL packet insertion

For DSI6G version 1.4.0 and above, for command mode use cases,
there is support in MDSS DSI to insert NULL packets automatically
between lines of a frame or High Speed DMA command burst to avoid
HS-LP-HS transitions in the middle of frame/DMA command transfer.
Add support for this in DSI driver.

Crs-Fixed: 880973
Change-Id: I468c2ea602d4c3efa0d8844c843840a15b631a3f
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>
Signed-off-by: Siddhartha Agrawal <agrawals@codeaurora.org>