drm/udl: Fixed problem with UDL adpater reconnection

Fixed problem with DisplayLink and DisplayLink certified adapers in drm/udl
driver when adapter doesn't want to work if it was initialized with
disconnected DVI cable by enabling drm connectot polling and updating
current connector's state.

Signed-off-by: Robert Tarasov <tutankhamen@chromium.org>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171013001350.172155-1-tutankhamen@chromium.org