drm/ttm: Delete an unnecessary check before the function call "ttm_tt_destroy"

The ttm_tt_destroy() function tests whether its argument is NULL
and then returns immediately. Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/54338f58-830c-a8b4-4554-5d4459bcd321@users.sourceforge.net

drivers/gpu/drm/ttm/ttm_bo.c

@@ -146,9 +146,7 @@ static void ttm_bo_release_list(struct kref *list_kref)
 	BUG_ON(bo->mem.mm_node != NULL);
 	BUG_ON(!list_empty(&bo->lru));
 	BUG_ON(!list_empty(&bo->ddestroy));
-
-	if (bo->ttm)
-		ttm_tt_destroy(bo->ttm);
+	ttm_tt_destroy(bo->ttm);
 	atomic_dec(&bo->glob->bo_count);
 	if (bo->resv == &bo->ttm_resv)
 		reservation_object_fini(&bo->ttm_resv);