drm/msm/dsi-staging: delete display nodes and use panel nodes

With the implementation of primary and secondary dsi node, display nodes
have become mostly redundant. Delete these display nodes and use panel
nodes directly. This simplifies the dsi probe and initialization.

Change-Id: I370cb5da3379cfd9fcdb36cf17485fe84f079b45
Signed-off-by: Wenjun Zhang <wjzhan@codeaurora.org>
Signed-off-by: Satya Rama Aditya Pinapala <psraditya30@codeaurora.org>