drm/omap: dss: Move connection checks to omapdss_device_(dis)connect

When a DSS output is (dis)connected the omapdss_output_(un)set_device()
function performs a sanity check to ensure that the output isn't already
(dis)connected. The check is unnecessary as those situations should
never happen, but can nonetheless be useful to catch driver bugs. To
prepare for removal of the omapdss_output_(un)set_device() functions
move the connection check to the omapdss_device_connect() function. The
omapdss_device_disconnect() already contains a corresponding check.

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>