drm/omap: Manage the usable omap_dss_device list within omap_drm_private

Instead of reaching back to DSS to iterate through the dss_devices every
time, use an internal array where we store the available and usable
dss_devices.

At the same time remove the omapdss_device_is_connected() check from
omap_modeset_init() as it became irrelevant: We are not adding dssdevs
if their connect failed.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
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>