ARM: dts: msm: primary disp pref for msm8974

Add primary display interface preference property
to mdss-mdp device node. This allows MDP to inform
to each controller if it is configured as the preferred
primary panel interface. Move the panels devices under
the mdss-mdp device node from soc since the panels will
no longer be considered a device. This change is required to
implement panel detection mechanism.

Change-Id: Id15cca1f0a4c768312a4d6ef37fac8064d4efb87
Signed-off-by: Manoj Rao <manojraj@codeaurora.org>