Create helper functions to use in computeInvariantOutput calls

src/effects/SkDisplacementMapEffect.cpp

Index: src/effects/SkDisplacementMapEffect.cpp
diff --git a/src/effects/SkDisplacementMapEffect.cpp b/src/effects/SkDisplacementMapEffect.cpp
index d137fba8c0e667f68860a92026ba3a6380f011bb..4068c909a2a88d8603f62c823cc0f8a920a6861b 100644
--- a/src/effects/SkDisplacementMapEffect.cpp
+++ b/src/effects/SkDisplacementMapEffect.cpp
@@ -497,8 +497,7 @@ void GrDisplacementMapEffect::onComputeInvariantOutput(InvariantOutput* inout) c
     // and no displacement offset push any texture coordinates out of bounds OR if the constant
     // alpha is 0. Since this isn't trivial to compute at this point, let's assume the output is
     // not of constant color when a displacement effect is applied.
-    inout->fValidFlags = 0;
-    inout->fIsSingleComponent = false;
+    inout->unknownOutput();
 }
 
 ///////////////////////////////////////////////////////////////////////////////