drm: omapdrm: connector-analog-tv: Remove unneeded check for OF node

As non-DT booting is no longer supported, the only way to instantiate
the device is through an OF node, which is guaranteed to be present.
Remove the unneeded check.

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