OMAPDSS: DPI: Use DPI driver data

DPI related data is currently a static global struct parameter. It is accessed
directly by functions in the driver.

This method won't work if we want the driver to support multiple DPI instances.
Create struct dpi_data, and pass its pointer to functions which need to use it.

We still have a static instance defined for dpi_data, which is accessed by top
level DPI ops. This will be removed when the driver dynamically allocates
dpi_data for each DPI instance.

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