ARM: dts: msm: enable variable refresh rate for 8939 QRD/MTP/CDP

Add DT property to enable variable refresh rate feature on NT35590
720p, NT35596 and JDI 1080p video mode panels on 8939 MTP, CDP
and QRD devices. This switches the fps of the panel by changing
the vertical front porch region keeping the DSI byte and pixel
clock rates constant.

Change-Id: I64d40babb43ec7c90c62737b0dee34cba80b0712
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>