drm/bridge: ps8640: Return an error for incorrect attach flags

Bridge drivers that implement the new model only shall return an error
from their attach() handler when the DRM_BRIDGE_ATTACH_NO_CONNECTOR flag
is not set. So make sure we return an error because only the new
drm_bridge model is supported.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Bilal Wasim <bwasim.lkml@gmail.com>
Tested-by: Bilal Wasim <bwasim.lkml@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826081526.674866-4-enric.balletbo@collabora.com