JNT_COMP: refactor if statements

Refactor if statement that use frame_offset == -1 to indicate
jnt_comp is not chosen, as distance now can not be negative.
Instead, add a variable use_jnt_comp_avg for the same functionality.

Change-Id: Ie6b9c6ab36131b48bc9e066babada17046729cd8
diff --git a/av1/common/convolve.h b/av1/common/convolve.h
index c9f6785..607532b 100644
--- a/av1/common/convolve.h
+++ b/av1/common/convolve.h
@@ -36,6 +36,7 @@
   int plane;
   int do_post_rounding;
 #if CONFIG_JNT_COMP
+  int use_jnt_comp_avg;
   int fwd_offset;
   int bck_offset;
 #endif