Vulkan: Minor cleanup of Transform Feedback validation

By using the helper isTransformFeedbackActive* functions.

Bug: angleproject:3205
Change-Id: I50b3e9db427da9f65c3e2b50d7847464e8c28202
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1665353
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
diff --git a/src/libANGLE/validationES2.cpp b/src/libANGLE/validationES2.cpp
index fc3a040..43cacf1 100644
--- a/src/libANGLE/validationES2.cpp
+++ b/src/libANGLE/validationES2.cpp
@@ -3453,9 +3453,9 @@
     ASSERT(buffer != nullptr);
 
     // Check if this buffer is currently being used as a transform feedback output buffer
-    TransformFeedback *transformFeedback = context->getState().getCurrentTransformFeedback();
-    if (transformFeedback != nullptr && transformFeedback->isActive())
+    if (context->getState().isTransformFeedbackActive())
     {
+        TransformFeedback *transformFeedback = context->getState().getCurrentTransformFeedback();
         for (size_t i = 0; i < transformFeedback->getIndexedBufferCount(); i++)
         {
             const auto &transformFeedbackBuffer = transformFeedback->getIndexedBuffer(i);