ARM: dts: qcom: add thermal cooling dev property under mdss_mdp on Holi

The display connector_init fails if the thermal cooling device
property "#cooling-cells" is not present under the mdss_mdp DT node.
Earlier this property used to be defined in "<target>-thermal-overlay.dtsi"
file, because of this, sometimes it gets missed.

Looking at the code of the sde_connector, it appears the "#cooling-cells"
is a mandatory property of the mdss_mdp.
Add this property under mdss_mdp DT node.

Change-Id: Icd4b076e0c718327df762f0dae10347162fa2299