drm/fsl-dcu: enable TCON bypass mode by default

Do not use encoder disable/enable callbacks to control bypass
mode as this seems to mess with the signals not liked by
displays. This also makes more sense since the encoder is
already defined to be parallel RGB/LVDS at creation time.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Tested-By: Meng Yi <meng.yi@nxp.com>