vc4: Detect working driver from cmdline

[Stefan Wahren]

Hi,
in order to improve kernelci.org results and avoid false positive cases like this [1], i suggested to also test for a working VC4 driver. In order to keep it simple, we should do it from userspace.

My first idea was:

test -d /sys/kernel/debug/dri/0 || echo "Error case"

But maybe you have a better / more reliable solution?

[1] - https://storage.kernelci.org/mainline/master/v4.17-rc5-69-g3acf4e395260/arm64/defconfig/lab-mhart/boot-bcm2837-rpi-3-b.html