Cleanup some of the Gaussian pyramid gen code

ScriptableRenderPipeline/HDRenderPipeline/HDRenderPipeline.cs

 
                 cmd.SetGlobalVector(HDShaderIDs._GaussianPyramidColorMipSize, new Vector4(pyramidSideSize, pyramidSideSize, lodCount, 0));
 
-                cmd.Blit(m_CameraColorBufferRT, m_GaussianPyramidColorBuffer);
+                cmd.Blit(m_CameraColorBufferRT, m_GaussianPyramidColorBufferRT);
-                colorPyramidDesc.depthBufferBits = 0;
-                colorPyramidDesc.colorFormat = RenderTextureFormat.ARGBHalf;
-                colorPyramidDesc.msaaSamples = 1;
+                colorPyramidDesc.useMipMap = false;
+
                 for (int i = 0; i < lodCount; i++)
                 {
                     colorPyramidDesc.width = colorPyramidDesc.width >> 1;
 
                 cmd.ReleaseTemporaryRT(HDShaderIDs._DepthPyramidMips[0]);
 
-                depthPyramidDesc.depthBufferBits = 0;
-                depthPyramidDesc.colorFormat = RenderTextureFormat.RFloat;
-                depthPyramidDesc.msaaSamples = 1;
+                depthPyramidDesc.useMipMap = false;
-                cmd.CopyTexture(HDShaderIDs._DepthPyramidMips[0], 0, 0, m_DepthPyramidBuffer, 0, 0);
+                cmd.CopyTexture(HDShaderIDs._DepthPyramidMips[0], 0, 0, m_DepthPyramidBufferRT, 0, 0);
 
                 for (int i = 0; i < lodCount; i++)
                 {