drm/bridge: it6505: Add caching for EDID

Add caching when EDID is read, and invalidate the cache until the
bridge detects HPD low or sink count changes on HPD_IRQ.

It takes 1.2s for IT6505 bridge to read a 3-block EDID, and skipping
one EDID read would be a notable difference on user experience.

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/20221115112720.911158-1-treapking@chromium.org