Currently the only users of drm_vblank_off() are i915 and gma500,
neither of which holds the event_lock when calling this function.
Fix this by holding the event_lock while traversing the list.
Signed-off-by: NImre Deak <imre.deak@intel.com>
Signed-off-by: NInki Dae <inki.dae@samsung.com>
Signed-off-by: NKyungmin Park <kyungmin.park@samsung.com>