Use COEFF_CONTEXT_MASK to replace hard coded number

Change-Id: Ib192e5e5feab3a87112389d15d88d1a0102b1f1a

av1/decoder/decodetxb.c

diff --git a/av1/decoder/decodetxb.c b/av1/decoder/decodetxb.c
index d0f24fd..bd1275f 100644
--- a/av1/decoder/decodetxb.c
+++ b/av1/decoder/decodetxb.c
@@ -296,7 +296,7 @@
     }
   }
 
-  cul_level = AOMMIN(63, cul_level);
+  cul_level = AOMMIN(COEFF_CONTEXT_MASK, cul_level);
 
   // DC value
   set_dc_sign(&cul_level, dc_val);