DRM: Don't re-poll connector for disconnect

DRM probe should not repoll a connector if it is already
connected and the DRM_CONNECTOR_POLL_DISCONNECT flag is not set.

Signed-off-by: Josef Holzmayr <holzmayr@rsi-elektrotechnik.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>