drm/mediatek: Only try to attach bridge if there is one

Even if dsi->bridge is NULL, we still try to call drm_bridge_attach,
and print out an error message, before creating the connector.

When no bridge is provided, let's skip these 2 steps and directly
create the connector.

Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Signed-off-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>