When cb4x4 is disabled and global-motion is enabled, the function
av1_append_sub8x8_mvs_for_idx accidentally fetched
cm->global_motion[ref] instead of cm->global_motion[rf[0]],
ie. it used the reference *index* instead of the reference *frame*.

Change-Id: Ieebf60f6b7c0a1cbbc083a70d4451ad799e27fe3