board: nitrogen6x: migrate to CONFIG_DM_VIDEO

Enable driver model for Video to remove compiler warning.
To enable display, issue

setenv stdout serial,vidconsole;

Add CONFIG_DM_VIDEO to all defconfigs whose board is
nitrogen6x.
        mx6qsabrelite_defconfig
        nitrogen6dl2g_defconfig
        nitrogen6dl_defconfig
        nitrogen6q2g_defconfig
        nitrogen6q_defconfig
        nitrogen6s1g_defconfig
	nitrogen6s_defconfig

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Tested-by: Anatolij Gustschin <agust@denx.de>