drm/omap: Remove connection checks from internal encoders .enable()

The internal encoders return an error from their .enable() handler when
their are not connected to a dss manager. As the flag used is set and
cleared in the connect and disconnect handlers, this effectively checks
whether the omap_dss_device is connected.

The .enable() handler is called from code paths that access the dss
devices chain from the display device, which is set to NULL when the
device isn't connected, making it impossible to access the device in
that case.

The safety check is thus not needed, remove it.

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