OMAPDSS: DSI: Remove dsi_pdev_map global struct

dsi_pdev_map is a struct visible globally in the DSI driver to get the platform
device pointer of the DSI device corresponding to it's module ID. This was
required because there was no clean way to derive the platform device from
the DSI module instance number or from the connected panel.

With the new output entity, it is possible to retrieve the platform device
pointer if the omap_dss_output pointer is available. Modify the functions
dsi_get_dsidev_from_dssdev() dsi_get_dsidev_from_id() so that they use output
instead of dsi_pdev_map to retrieve the dsi platform device pointer.

Signed-off-by: Archit Taneja <archit@ti.com>