configs: pico-imx6ul: convert DM_VIDEO

It's due to the warning messages issue after compiled:

  ===================== WARNING ======================
  This board does not use CONFIG_DM_VIDEO Please update
  the board to use CONFIG_DM_VIDEO before the v2019.07 release.
  Failure to update by the deadline may result in board removal.
  See doc/driver-model/migration.rst for more info.
  ====================================================

Enable DM_VIDEO can fix it on the stage of compilation.

Signed-off-by: Wig Cheng <wig.cheng@technexion.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>