drm/omap: dss: Move common device operations to common structure

The various types of omapdss_*_ops structures define multiple operations
that are not specific to a bus type. To simplify the code and remove
dependencies on specific bus types move those operations to a common
structure. Operations that are specific to a bus type are kept in the
specialized ops structures.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>