drm/xe: Fix build with KUNIT=m

Due to the current integration between "live" xe kunit tests and kunit,
it's not possible to have a build with the following combination:

	CONFIG_DRM_XE=y
	CONFIG_KUNIT=m

... even if kconfig doesn't block it. The reason for the failure is that
some compilation units are pulled in xe.ko:

	drivers/gpu/drm/xe/xe_bo.c:#include "tests/xe_bo.c"
	drivers/gpu/drm/xe/xe_dma_buf.c:#include "tests/xe_dma_buf.c"
	drivers/gpu/drm/xe/xe_migrate.c:#include "tests/xe_migrate.c"
	drivers/gpu/drm/xe/xe_pci.c:#include "tests/xe_pci.c"

Those files shouldn't use symbols from kunit, which should be reserved
to the tests/*_test.c files. Detangling this dependency doesn't seem
very straightforward, so fix the immediate issue instructing kconfig to
block the problematic configuration.
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20231109175132.3084142-3-lucas.demarchi@intel.comSigned-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/xe/Kconfig

 # SPDX-License-Identifier: GPL-2.0-only
 config DRM_XE
 	tristate "Intel Xe Graphics"
-	depends on DRM && PCI && MMU
+	depends on DRM && PCI && MMU && (m || (y && KUNIT=y))
 	select INTERVAL_TREE
 	# we need shmfs for the swappable backing store, and in particular
 	# the shmem_readpage() which depends upon tmpfs