drm/bridge/lontium-lt9611uxc: fix provided connector suport

- set DRM_CONNECTOR_POLL_HPD as the connector will generate hotplug
  events on its own

- do not call drm_kms_helper_hotplug_event() unless mode_config.funcs
  pointer is not NULL to remove possible kernel oops.

Fixes: bc6fa867

 ("drm/bridge/lontium-lt9611uxc: move HPD notification out of IRQ handler")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708230329.395976-1-dmitry.baryshkov@linaro.org