ARM: dts: msm: enable double buffering mode support for DSI-1 on 8939

On 8916 and 8939 DSI controller supports double buffered timing
mode. Enabling the same was missed in mdss dtsi file for DSI-1
on 8939. Add the DT entry to enable it for DSI-1.

Change-Id: I378893784a599bac57585a128bf51a448260faf1
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>