drm/omap: remove legacy DSS device operations

All DSS devices have been converted to bridge API, so
the device operations are always NULL. This removes
the device ops function pointers and all code using it.

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/20201215104657.802264-42-tomi.valkeinen@ti.com