unify with bigTileIdx code from lightlistbuild-clustered

Assets/ScriptableRenderPipeline/HDRenderPipeline/Lighting/TilePass/Resources/lightlistbuild.compute

 
 	// build coarse list using AABB
 #ifdef USE_TWO_PASS_TILED_LIGHTING
-	int NrBigTilesX = (nrTilesX+3)>>2;
-	const int bigTileIdx = (tileIDX.y>>2)*NrBigTilesX + (tileIDX.x>>2);		// map the idx to 64x64 tiles
+	const uint log2BigTileToTileRatio = firstbithigh(64) - firstbithigh(16);
+
+	int NrBigTilesX = (nrTilesX+((1<<log2BigTileToTileRatio)-1))>>log2BigTileToTileRatio;
+	const int bigTileIdx = (tileIDX.y>>log2BigTileToTileRatio)*NrBigTilesX + (tileIDX.x>>log2BigTileToTileRatio);		// map the idx to 64x64 tiles
 	int nrBigTileLights = g_vBigTileLightList[MAX_NR_BIGTILE_LIGHTS_PLUSONE*bigTileIdx+0];
 	for(int l0=(int) t; l0<(int) nrBigTileLights; l0 += NR_THREADS)
 	{

Assets/ScriptableRenderPipeline/fptl/lightlistbuild.compute

 
 	// build coarse list using AABB
 #ifdef USE_TWO_PASS_TILED_LIGHTING
-	int NrBigTilesX = (nrTilesX+3)>>2;
-	const int bigTileIdx = (tileIDX.y>>2)*NrBigTilesX + (tileIDX.x>>2);		// map the idx to 64x64 tiles
+	const uint log2BigTileToTileRatio = firstbithigh(64) - firstbithigh(16);
+
+	int NrBigTilesX = (nrTilesX+((1<<log2BigTileToTileRatio)-1))>>log2BigTileToTileRatio;
+	const int bigTileIdx = (tileIDX.y>>log2BigTileToTileRatio)*NrBigTilesX + (tileIDX.x>>log2BigTileToTileRatio);		// map the idx to 64x64 tiles
 	int nrBigTileLights = g_vBigTileLightList[MAX_NR_BIGTILE_LIGHTS_PLUSONE*bigTileIdx+0];
 	for(int l0=(int) t; l0<(int) nrBigTileLights; l0 += NR_THREADS)
 	{