drm/omap: Remove duplicate declaration

struct dss_device has been declared. Remove the duplicate.
And sort these forward declarations alphabetically.

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210325111028.864628-1-wanjiabing@vivo.com