msm: mdss: add support to featurize 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. Add support to featurize this based on DT
property.

Crs-Fixed: 880973
Change-Id: I36acfe6d27a1d750a5af12c3d136df473abb4598
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>
Signed-off-by: Siddhartha Agrawal <agrawals@codeaurora.org>