drm/bridge: sn65dsi86: defer if there is no dsi host

Otherwise we don't get another shot at it if the bridge probes before
the dsi host is registered.  It seems like this is what *most* (but not
all) of the other bridges do.

It looks like this was missed in the conversion to attach dsi host at
probe time.

Fixes: c3b75d47

 ("drm/bridge: sn65dsi86: Register and attach our DSI device at probe")
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Tested-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
[dianders: squashed in Stephen's simplification]
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211207215753.635841-1-robdclark@gmail.com