OMAPDSS: DPI: Allocate driver data

Allocate driver data(dpi_data) for each DPI instance. It's allocated in
omap_dpi_probe() when DT isn't used, and in dpi_init_port() when DT is used.
The dpi_data struct instance is no longer global. In the case of DPI ops, it's
retrieved from dpi_get_data_from_dssdev(). 'dssdev' passed by the connected
encoder/panel driver is a pointer to the 'output' member in dpi_data, and thus
can be used to get the DPI instance's driver data. In the case of probe/ini_port
functions, it's set as DPI/DSS device's private data embedded in the
platform_device struct.

Having dpi_data as private data of the platform device will not work for
multiple DPI instances in the DT case. This is because there is no corresponding
platform_device for DPI or SDI, they exist only as ports under the parent DSS
platform_device in the DT case. The DPI port's private data('data' member in
device_node struct) will later be used to store dpi_data.

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