drm/bridge: it6505: Improve synchronization between extcon subsystem

Originally, the it6505 relies on a short sleep in the IRQ handler and a
long sleep to make sure it6505->lane_swap and it6505->lane_count is
configured in it6505_extcon_work and it6505_detect, respectively.

Use completion and additional DPCD read to remove the unnecessary waits,
and use a different lock for it6505_extcon_work and the threaded IRQ
handler because they no longer need to run exclusively.

The wait time of the completion is usually less than 10ms in local
experiments, but leave it larger here just in case.

Signed-off-by: Pin-yen Lin <treapking@chromium.org>
Reviewed-by: Robert Foss <robert.foss@linaro.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221013110411.1674359-4-treapking@chromium.org