drm/xe: Use symbol namespace for kunit tests

Instead of simply using EXPORT_SYMBOL() to export the functions needed
in xe.ko to be be called across modules, use EXPORT_SYMBOL_IF_KUNIT()
which will export the symbol under the EXPORTED_FOR_KUNIT_TESTING
namespace.

This avoids accidentally "leaking" these functions and letting them be
called from outside the kunit tests. If these functiosn are accidentally
called from another module, they receive a modpost error like below:

	ERROR: modpost: module XXXXXXX uses symbol
	xe_ccs_migrate_kunit from namespace EXPORTED_FOR_KUNIT_TESTING,
	but does not import it.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/20230401085151.1786204-4-lucas.demarchi@intel.com