Bring back HDMI

Epic: https://source.puri.sm/Librem5/use-cases/issues/108

Our current drm code does not carry the HDMI code from NXP.

changed the description

at least we include Guido's hdmi work starting with kernel 5.6. It's not on by default or working alongside the internal screen yet AFAIK.

we have a separate DT for HDMI on the devkit so closing.

closed