ARM: dts: msm: Add CX regulator Phandle to MDP node

The Phandle might change for different versions of
the msm8974 chip. Add the proper Phandle of CX regulator
to 8974pro version.

Change-Id: I6e8ab19c9765ff225a54a621cd001e8af1c7391d
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>