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