gitlab-ci: compile Linux image with support to VGEM

Add VGEM to the Linux image that runs in the CI. There are tests that we
plan to add in the future that need this.

This brings a complication, as we already have VKMS in the image. The
order in which DRM devices are loaded is not always the same, so the
node they receive is non-deterministic. Until now we were sure that VKMS
(the virtual device we use to run the DRM-backend tests in the CI) would
be in "/dev/dri/card0", but now we can't be sure. To deal with this
problem we find the node of each device using a one-liner shell script.

This commit also updates the documentation section that describes
specificities of DRM-backend tests in our test suite.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>