drm/xe/display: Add empty def for i915_gem_object_flush_if_display

We don't need i915_gem_object_flush_if_display on Xe side. Add empty
define to tackle compilation errors with display code where it's used.
Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/xe/xe_bo.h

@@ -325,6 +325,8 @@ static inline unsigned int xe_sg_segment_size(struct device *dev)
 	return round_down(max / 2, PAGE_SIZE);
 }
 
+#define i915_gem_object_flush_if_display(obj)		((void)(obj))
+
 #if IS_ENABLED(CONFIG_DRM_XE_KUNIT_TEST)
 /**
  * xe_bo_is_mem_type - Whether the bo currently resides in the given