Core Changes:

- fence: Introduce new fence flag to signify timestamp is populated (Chris)
- mst: Avoid processing incomplete data + fix NULL dereference (Imre)

Driver Changes:
- vc4: Avoid WARN from grabbing a ref from vblank that's not on (Boris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Imre Deak <imre.deak@intel.com>