drm/bridge: ps8640: Return NULL immediately when EDID read fail

drm_edid_read returns NULL on error, so feeding it directly into
drm_edid_duplicate may lead to NULL pointer dereference. Add a check to
guard this.

Fixes: 6a17b4d1

 ("drm/bridge: ps8640: Add a cache for EDID")
Signed-off-by: Pin-yen Lin <treapking@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230324035909.1727356-1-treapking@chromium.org