ARM: dts: msm: display: enable pingpong-split support for msm8996

Pingpong-split is a data path configuration where one layer mixer is
used to drive dual-DSI panels. Enable it so it can be used if needed.

Change-Id: Ia0fbe3e73f1fa534f42503b7c632ec7cfdb16685
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>