drm/xe/Kconfig.debug: select DEBUG_FS for KUnit runs

KUnit reuquires debugfs, as otherwise, it won't build:

$ make ARCH=x86_64 O=.kunit --jobs=8
ERROR:root:../drivers/gpu/drm/xe/display/intel_display_debugfs.c:1612:6: error: redefinition of ‘intel_display_debugfs_register’
 1612 | void intel_display_debugfs_register(struct drm_i915_private *i915)
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from ../drivers/gpu/drm/xe/display/intel_display_debugfs.c:18:
../drivers/gpu/drm/xe/display/intel_display_debugfs.h:18:20: note: previous definition of ‘intel_display_debugfs_register’ with type ‘void(struct xe_device *)’
   18 | static inline void intel_display_debugfs_register(struct drm_i915_private *i915) {}
      |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../drivers/gpu/drm/xe/display/intel_display_debugfs.c:1935:6: error: redefinition of ‘intel_connector_debugfs_add’
 1935 | void intel_connector_debugfs_add(struct intel_connector *intel_connector)
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~
../drivers/gpu/drm/xe/display/intel_display_debugfs.h:19:20: note: previous definition of ‘intel_connector_debugfs_add’ with type ‘void(struct intel_connector *)’
   19 | static inline void intel_connector_debugfs_add(struct intel_connector *connector) {}
      |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~
../drivers/gpu/drm/xe/display/intel_display_debugfs.c:1993:6: error: redefinition of ‘intel_crtc_debugfs_add’
 1993 | void intel_crtc_debugfs_add(struct drm_crtc *crtc)
      |      ^~~~~~~~~~~~~~~~~~~~~~
../drivers/gpu/drm/xe/display/intel_display_debugfs.h:20:20: note: previous definition of ‘intel_crtc_debugfs_add’ with type ‘void(struct drm_crtc *)’
   20 | static inline void intel_crtc_debugfs_add(struct drm_crtc *crtc) {}
      |                    ^~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>