drm/msm/dsi-staging: Add support for TE control

Panel TE control is defined in the panel dtsi. DSI controller is
required to setup the TE path in the command trigger control. This
change will make sure the TE control is read from the panel dtsi.

Change-Id: I88f9464ee96b93efd27daf74b73d8d3d74538e2e
Signed-off-by: Benjamin Chan <bkchan@codeaurora.org>