drm/i915/gt: fix spelling mistake "undeflow" -> "underflow"

There is a spelling mistake in comment, fix it.
Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200401022506.52965-1-chenzhou10@huawei.com

drivers/gpu/drm/i915/gt/intel_engine_pm.c

@@ -181,7 +181,7 @@ static bool switch_to_kernel_context(struct intel_engine_cs *engine)
 	 * Ergo, if we put ourselves on the timelines.active_list
 	 * (se intel_timeline_enter()) before we increment the
 	 * engine->wakeref.count, we may see the request completion and retire
-	 * it causing an undeflow of the engine->wakeref.
+	 * it causing an underflow of the engine->wakeref.
 	 */
 	flags = __timeline_mark_lock(ce);
 	GEM_BUG_ON(atomic_read(&ce->timeline->active_count) < 0);