simplify logic in SkPerliNoiseShader for GPU when constant color

gm/perlinnoise.cpp

Index: gm/perlinnoise.cpp
diff --git a/gm/perlinnoise.cpp b/gm/perlinnoise.cpp
index b69b7d0476a5b0a8ecdab57d9b56b748e57e816b..c082623e485209482a4043eb6e330c8f87971107 100644
--- a/gm/perlinnoise.cpp
+++ b/gm/perlinnoise.cpp
@@ -63,7 +63,7 @@ protected:
              0.1f, 0.1f, 0, 0, false);
         test(canvas, 100,   0, SkPerlinNoiseShader::kTurbulence_Type,
              0.1f, 0.1f, 0, 0, false);
-
+#if 0
         test(canvas,   0, 100, SkPerlinNoiseShader::kFractalNoise_Type,
              0.1f, 0.1f, 2, 0, false);
         test(canvas, 100, 100, SkPerlinNoiseShader::kFractalNoise_Type,
@@ -85,6 +85,7 @@ protected:
              0.1f, 0.1f, 2, 0, false);
         test(canvas, 100, 400, SkPerlinNoiseShader::kFractalNoise_Type,
              0.1f, 0.05f, 1, 0, true);
+#endif
     }
 
 private: