av1_find_mv_refs sets the ALL_ZERO flag when either (a) we don't have
enough candidate MVs, or (b) all candidate MVs are zero. With global
motion enabled, case (b) does not work as intended and causes two
problems: a significant quality decrease for foreman_cif.y4m, and
test failures in *CpuSpeedTest*Screencast*.

This patch skips check (b) when global motion is enabled, fixing
the above bugs.

Change-Id: I7461588cb02733563c1439f430b89190299a1b5d