drm/i915/execlists: Only mark incomplete requests as -EIO on cancelling

Only the requests that have not completed do we want to change the
status of to signal the -EIO when cancelling the inflight set of requests
upon wedging.

Reported-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191011103345.26013-1-chris@chris-wilson.co.uk