Minor refactor jnt_comp distance value

Instead of using -1 to indicate jnt_comp is used or not, use a separate
variable.

Change-Id: I9b206d63394c431741d89611a614087343a417fc
diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 90aa9a7..e89713d 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -5860,8 +5860,7 @@
     const int plane = 0;
     ConvolveParams conv_params = get_conv_params(!id, 0, plane);
 #if CONFIG_JNT_COMP
-    conv_params.fwd_offset = -1;
-    conv_params.bck_offset = -1;
+    conv_params.use_jnt_comp_avg = 0;
 #endif
     WarpTypesAllowed warp_types;
     warp_types.global_warp_allowed = is_global[!id];