.gitlab.ci: Enable debug for libsteat and for the DRM backend

As the drm-smoke test randomly reports having the connector disabled,
and with it libseat reports setMaster errors, this enables DRM backend
debug messages for the kernel, and for libseat in an attempt to
track down the issue, whenever it might happen again.

These are pretty harmless, in terms of data being generated as we only
have a single DRM test using VKMS, and the libseat debug message aren't
that verbose, so we're safe keeping them for the time being.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>