ARM: dts: msm: update ext dsi bridge display

Use phandle as the reference to ext dsi bridge node.
The existing way, "qcom,dsi-display@17", is prone to
conflict when adding new dsi display.

CRs-Fixed: 2300942
Signed-off-by: Chong Gu <chongu@codeaurora.org>
Change-Id: Iff56291150e77054a035e0816f31190036e210f2