Fixed the number of sample for GGX convolution.

Assets/ScriptableRenderLoop/HDRenderLoop/Sky/Resources/GGXConvolve.shader

                                             PerceptualRoughnessToRoughness(perceptualRoughness),
                                             _MipMapCount,
                                             _InvOmegaP,
-                                            1,
+                                            2048,
                                             false);
 
                 return val;