drm/i915: Fix the flip synchronisation to consider mmioflips

Currently we emit semaphore synchronisation as if we were going to flip
using the target CS engine, but we then change our minds and do the flip
using the CPU. Consequently we write instructions to the ring but never
use them - even to the point of filling that ring up entirely and never
submitting a request.

The wrinkle in the ointment is that we have to tell a white lie to
pin-to-display for it to skip the synchronisation for mmioflips as we
will create a task specifically for that slow synchronisation. An oddity
of note is the discrepancy in requests that we tell to pin-display to
serialise to and that we then eventually wait upon. This is due to a
limitation in the i915_gem_object_sync() routine that will be lifted
later.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>