drm/omap: dsi: simplify pin config

Simplify DSI pin config, which always originates from DT
nowadays. With the code being fully contained in the DSI
encoder, we can drop the public structure.

Since the function is no longer exposed, it now directly
takes the private DSI data pointer. This drops a pointless
conversion and means the pins can be configured earlier.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-49-tomi.valkeinen@ti.com