gitlab-ci: Build kernel with udmabuf support

With udmabuf we can write test for dmabuf paths in a similar
manner to shm. It's available by default in most recent distros
and thus works OOTB on developer setups.

Drop vgem as it isn't needed any more and makes the CI use
kms_swrast (instead swrast), which isn't compatible with the
llvmpipe dmabuf implementation yet.
Using swrast also streamlines CI with running tests locally,
making debugging easier.

Signed-off-by: Robert Mader <robert.mader@collabora.com>