OMAPDSS: DPI: Add support for multiple instances

Register DPI outputs, and assign the port_num to them as specified by the
'reg' property in the DPI ports in DT.

To support multiple DPI instances, dpi_get_channel needs to take the DPI
instance's port number to get the corresponding channel. Make it take this
argument. We just pass 0 in the non-DT path, since we don't support multiple
instances in the non-DT case.

Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>