Fix compile error for global-motion without ref-mv

The reference frame handling in av1_find_mv_refs introduced
in https://aomedia-review.googlesource.com/c/6067/ broke the
compile when global-motion is enabled but ref-mv is not.
This patch adds the missing logic, allowing this case to compile
again.

Change-Id: I914887eb56d28a700b2917d086447bdbb314f35d

av1/common/mvref_common.c

diff --git a/av1/common/mvref_common.c b/av1/common/mvref_common.c
index f83f98d..39a8cb4 100644
--- a/av1/common/mvref_common.c
+++ b/av1/common/mvref_common.c
@@ -874,10 +874,10 @@
   int_mv zeromv[2];
 #if CONFIG_REF_MV
   int idx, all_zero = 1;
-#endif
 #if CONFIG_GLOBAL_MOTION
   MV_REFERENCE_FRAME rf[2];
 #endif
+#endif
 #if CONFIG_EXT_INTER
   av1_update_mv_context(xd, mi, ref_frame, mv_ref_list, -1, mi_row, mi_col,
 #if CONFIG_REF_MV
@@ -888,6 +888,7 @@
 #endif  // CONFIG_EXT_INTER
 
 #if CONFIG_GLOBAL_MOTION
+#if CONFIG_REF_MV
   av1_set_ref_frame(rf, ref_frame);
   zeromv[0].as_int = gm_get_motion_vector(&cm->global_motion[rf[0]],
                                           cm->allow_high_precision_mv)
@@ -898,8 +899,14 @@
                                .as_int
                          : 0;
 #else
+  zeromv[0].as_int = gm_get_motion_vector(&cm->global_motion[ref_frame],
+                                          cm->allow_high_precision_mv)
+                         .as_int;
+  zeromv[1].as_int = 0;
+#endif  // CONFIG_REF_MV
+#else
   zeromv[0].as_int = zeromv[1].as_int = 0;
-#endif
+#endif  // CONFIG_GLOBAL_MOTION
 
 #if CONFIG_REF_MV
   if (ref_frame <= ALTREF_FRAME)