drm/omap: Remove connection checks from display .enable() and .remove()

The displays (connectors, panels and encoders) return an error from
their .enable() handler when the dss device is not connected. They also
disconnect the dss device explicitly from their .remove() handler if it
is still connected.

Those safety checks are not needed:

- 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.

- The .remove() handler can only be called when unloading the module as
  the driver has the suppress_bind_attrs attribute set, and a reference
  to the module is taken when constructing the dss devices chain, so the
  module can only be unloaded when the dss device is disconnected.

Remove the safety checks.

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>