drm/omap: dss: Create global list of all omap_dss_device instances

The omap_dss_device instances are stored in two separate lists,
depending on whether they are panels or outputs. Create a third list
that stores all omap_dss_device instances to allow generic code to
operate on all instances.

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>