ARM: dts: msm: primary disp pref for msm8226

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: Ic94367a4848c3020fdab1cda2830040832f99cde
Signed-off-by: Manoj Rao <manojraj@codeaurora.org>