drm/i915: Invalidate the to-ring, flush the old-ring when updating domains

When the object has been written to by the gpu it remains on the ring
until its flush has been retired. However, when the object is moving to
the ring and the associated cache needs to be invalidated, we need to
perform the flush on the target ring, not the one it came from (which is
NULL in the reported case and so the flush was entirely absent).

Reported-by: Peter Clifton <pcjc2@cam.ac.uk>
Reported-and-tested-by: Alexey Fisher <bug-track@fisher-privat.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>