Enable DM Video support on iMX6Q Novena and fix minor details
to restore previous behavior of the system.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Vagrant Cascadian <vagrant@debian.org>