drm/udl: Remove DRM_CONNECTOR_POLL_HPD

DisplayLink devices do not generate hotplug events. Remove the poll
flag DRM_CONNECTOR_POLL_HPD, as it may not be specified together with
DRM_CONNECTOR_POLL_CONNECT or DRM_CONNECTOR_POLL_DISCONNECT.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: afdfc4c6

 ("drm/udl: Fixed problem with UDL adpater reconnection")
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Cc: Robert Tarasov <tutankhamen@chromium.org>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Sean Paul <sean@poorly.run>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v4.15+
Link: https://patchwork.freedesktop.org/patch/msgid/20240510154841.11370-2-tzimmermann@suse.de