ARM: dts: msm: Enable Control Path and mixer preference on msm8996

Add dtsi entries for display control path and mixer destination display
preference.  The possible input are "primary", "secondary", "tertiary",
"none".  For example, when user set ctrl path 1 preference to
"primary" and mixer1 preference to "primary".  SDE RM will let primary
display to use ctrl path 1 and mixer 1.

Change-Id: Ie7f0cb122552a3d8b7406a8d1fcbcb184c67a073
Signed-off-by: Camus Wong <camusw@codeaurora.org>