So far we've been able to use the official 7' touchscreen using the frame
buffer device provided by UEFI. This comes with the downside that VC4 has to be
disabled in order for the setup to work. 

The alternative is to use VC4 and the raspberrypi-panel driver. This comes with
some downsides: the setup depends on an overlay which isn't provided by anyone
(Raspbian uses a custom firmware based frame buffer device) and the back-light
is operated through the I2C0 bus, which is owned by the firmware.

We should work towards getting the firmware based back-light implementation
into the upstream kernel and with that discuss on integrating a suitable
overlay into raspbian's kernel tree.