Disable HDMI in u-boot for rk3399 boards

One of the change in u-boot v2020.04 was the enablement of HDMI in u-boot.
I tested this scenario while upgrading u-boot with WQHD Dell u2719d which I have access to and it was working.

However since the switch to u-boot v2020.04 there are two reported cases already that users cannot boot with v20.05.x and HDMI cable connected during boot.

• https://forum.armbian.com/topic/14289-nanopi-m4-v1-wont-boot-latest-versions/

• https://forum.armbian.com/topic/13865-nanopc-t4-will-not-boot-image-made-with-current-build-script-bug/

• Unplugging HDMI cable for boot and plugging it after boot helps.