drm/i915: Confirm request->global_seqno after spin completion

drm/i915: Confirm request->global_seqno after spin completion

After we see our target seqno has been completed by the hw, we need to
confirm that it still matches the request (as it may have been preempted
before the spin completes). If the request no longer matches the target
seqno, we need to restart the wait to reacquire that seqno.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170921210903.18337-1-chris@chris-wilson.co.uk
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>