drm/xe: Add kernel doc for xe_hw_engine_lookup

kerneldoc was missing from earlier commit where
we exported xe_hw_engine_lookup. Add it.

Cc: Dominik Grzegorzek <dominik.grzegorzek@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>
Cc: intel-xe@lists.freedesktop.org
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240806153009.1081382-2-mika.kuoppala@linux.intel.com

drivers/gpu/drm/xe/xe_hw_engine.c

@@ -1173,6 +1173,16 @@ static const enum xe_engine_class user_to_xe_engine_class[] = {
 	[DRM_XE_ENGINE_CLASS_COMPUTE] = XE_ENGINE_CLASS_COMPUTE,
 };
 
+/**
+ * xe_hw_engine_lookup() - Lookup hardware engine for class:instance
+ * @xe: xe device
+ * @eci: engine class and instance
+ *
+ * This function will find a hardware engine for given engine
+ * class and instance.
+ *
+ * Return: If found xe_hw_engine pointer, NULL otherwise.
+ */
 struct xe_hw_engine *
 xe_hw_engine_lookup(struct xe_device *xe,
 		    struct drm_xe_engine_class_instance eci)