drm/bridge/lontium-lt9611uxc: fix waiting for EDID to become available

- Call wake_up() when EDID ready event is received to wake
  wait_event_interruptible_timeout()

- Increase waiting timeout, reading EDID can take longer than 100ms, so
  let's be on a safe side.

- Return NULL pointer from get_edid() callback rather than ERR_PTR()
  pointer, as DRM code does NULL checks rather than IS_ERR().

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>