drm/omap: dss: Merge two disconnection helpers

To simplify the pipeline disconnection handling merge the
omapdss_device_disconnect() and omapdss_output_unset_device() functions.
The device state check is now called for every device in the pipeline,
extending this sanity check coverage.

There is no need to return an error from omapdss_device_disconnect()
when the check fails, as omapdss_output_unset_device() used to do, given
that we can't prevent disconnection due to device unbinding (the return
value of omapdss_output_unset_device() is never checked in the current
code for that reason).

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>