drm/omap: Remove unneeded safety checks in the HPD operations

The HPD-related omap_dss_device operations are now only called when the
device supports HPD. There's no need to duplicate that check in the
omap_dss_device drivers. The .register_hpd_cb() operation can as a
result be turned into a void operation.

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>