pico-imx6ul: disable video after DM_VIDEO conversion deadline

These boards were not converted to DM_VIDEO before deadline,
so disable video support for now.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Richard Hu <richard.hu@technexion.com>
Cc: Fabio Estevam <festevam@gmail.com>