ARM: dts: msm: add support for NULL packet insertion for MSM8996

Add the DT property to enable the NULL packet insertion feature
for MSM8996 target. This helps to avoid LP-HS-LP transitions during
pixel data or DMA packet transfer for command mode panels.

Crs-Fixed: 880973
Change-Id: I512ac7c57093b7f4cb7048e9fbb87baa113eedb2
Signed-off-by: Siddhartha Agrawal <agrawals@codeaurora.org>