Corrected parameters to tex.GetDimensions in GetMipCount

(no resulting functional change)

ScriptableRenderPipeline/Core/CoreRP/ShaderLibrary/Common.hlsl

     // Metal doesn't support high enough OpenGL version
 
 #if defined(MIP_COUNT_SUPPORTED)
-    uint width, height, depth, mipCount;
-    width = height = depth = mipCount = 0;
-    tex.GetDimensions(width, height, depth, mipCount);
-    return mipCount;
+    uint mipLevel, width, height, mipCount;
+    mipLevel = width = height = mipCount = 0;
+	tex.GetDimensions(mipLevel, width, height, mipCount);
+    return mipCount;	
 #else
     return 0;
 #endif