[normative]Reset film grain params if monochrome

BUG=aomedia:2050

Change-Id: I39ab22e7710036b9cdc8e2b4b70c47e93e11d337

av1/decoder/decodeframe.c

diff --git a/av1/decoder/decodeframe.c b/av1/decoder/decodeframe.c
index 6cce416..952669c 100644
--- a/av1/decoder/decodeframe.c
+++ b/av1/decoder/decodeframe.c
@@ -3910,6 +3910,8 @@
 
   if (!seq_params->monochrome)
     pars->chroma_scaling_from_luma = aom_rb_read_bit(rb);
+  else
+    pars->chroma_scaling_from_luma = 0;
 
   if (seq_params->monochrome || pars->chroma_scaling_from_luma ||
       ((seq_params->subsampling_x == 1) && (seq_params->subsampling_y == 1) &&