drm/omap: Merge HPD enable operation with HPD callback registration

The omap_dss_device .enable_hpd() and .disable_hpd() are used to enable
and disable hot-plug detection at omapdrm probe and remove time. This is
required to avoid reporting hot-plug detection events before the DRM
infrastructure is ready to accept them, as that could result in crashes
or other malfunction.

Hot-plug event reporting is conditioned by both HPD being enabled
through the .enable_hpd() operation and by the HPD callback being
registered though the .register_hpd_cb() operation. We thus don't need a
separate enable operation if we can guarantee that callbacks won't be
registered too early.

HPD callbacks are registered at connector initialization time, which is
too early to start reporting HPD events. There's however nothing
blocking a move of callback registration to a later time when the
omapdrm driver calls the HPD enable operations. Do so, and remove the
HPD enable operation completely from omap_dss_device drivers.

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>