panel-simple.c is a driver for a collection of RGB panels (mipi DPI) and DSI panels (mipi DSI). The dsi_of_match table is to be used when you have a DSI panel, and the other for RGB panels.
The dtsi looks okay, and so do the changes in panel-simple.c. Can you put some prints in the panel-simple.c driver's probe and see if it is successfully reaching mipi_dsi_attach?
If this happens successfully, the msm's dsi_host driver's callback "dsi_host_attach" (in drivers/gpu/drm/msm/dsi/dsi_host.c) will be called successfully.
The issue you observe happens when the panel isn't successfully registered.