drm/xe: Allow dropping kunit dependency as built-in

drm/xe: Allow dropping kunit dependency as built-in

Fix Kconfig symbol dependency on KUNIT, which isn't actually required
for XE to be built-in. However, if KUNIT is enabled, it must be built-in
too.

Fixes: 08987a8b6820 ("drm/xe: Fix build with KUNIT=m")
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Harry Austen <hpausten@protonmail.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20250627-xe-kunit-v2-2-756fe5cd56cf@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>