drm/xe: Add min config for kunit integration ARCH=um

Some of the tests may benefit from running with ARCH=um, forgoing any
additional setup on the CI build side. Add min config for that.

Tested with:

	./tools/testing/kunit/kunit.py build \
		--kunitconfig drivers/gpu/drm/xe/.kunitconfig \
		--jobs $(nproc) \
		--build_dir build_kunit

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org>