[CFL] Limit Luma Partition to 32X32

Based on the HW Subgroup call of December 4th 2017, we limit luma partition to
32X32.

Regression on Subset 1
  PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
0.0881 |  1.3504 |  1.2936 |   0.0572 |  0.0182 |  0.0227 |     0.5204

https://two.arewecompressedyet.com/?job=CfL-PartU%402017-12-12T15%3A39%3A36.794Z&job=CfL-Max32x32%402017-12-12T16%3A10%3A09.989Z

Change-Id: I7e3cfd68097c0bc24b1426348b5fd574c4f638a0
diff --git a/av1/common/blockd.h b/av1/common/blockd.h
index 24be7d9..d5c5426 100644
--- a/av1/common/blockd.h
+++ b/av1/common/blockd.h
@@ -568,6 +568,7 @@
 #define CFL_SUB8X8_VAL_MI_SQUARE \
   (CFL_SUB8X8_VAL_MI_SIZE * CFL_SUB8X8_VAL_MI_SIZE)
 #endif  // CONFIG_DEBUG
+#define CFL_MAX_BLOCK_SIZE (BLOCK_32X32)
 typedef struct cfl_ctx {
   // The CfL prediction buffer is used in two steps:
   //   1. Stores Q3 reconstructed luma pixels