drm/omap: drop unused dsi.configure_pins

The panel-dsi-cm's ddata->pin_config is always NULL, so this
callback is never called. Instead the DSI encoder gets the pin
configuration directly from DT.

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/20201215104657.802264-3-tomi.valkeinen@ti.com