drm/bridge: ps8640: Add a cache for EDID

When there are multiple EDID reads, the bridge will be repeatedly
enabled and disabled. Add a cache for EDID to speed this up.

Signed-off-by: Pin-yen Lin <treapking@chromium.org>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230315035508.2874915-2-treapking@chromium.org