colibri-imx6ull: support building with DM_VIDEO=y

1. This fixes linking issues when building with DM_VIDEO enabled mxsfb
driver.
2. Provide proper defines for both VIDEO=y and DM_VIDEO=y.

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>