Revert "drm/omap: dss: Remove unused omap_dss_device operations"

This reverts commit e086558a ("drm/omap: dss: Remove unused
omap_dss_device operations")

This is still needed by DSI. E.g. unloading modules without this will
cause a crash.

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/20201215104657.802264-2-tomi.valkeinen@ti.com