drm/i915/selftests: Mark contexts as lost during freeing of mock device

We need to unpin the last retired context early in the shutdown sequence
so that its RCU free is done before we try to free the context ida. I
included this in a later patch ("drm/i915: Keep a recent cache of freed
contexts objects for reuse") and so missed that the selftests were broken
in the meantime.

Reported-by: Matthew Auld <matthew.auld@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101627


Fixes: 5f09a9c8 ("drm/i915: Allow contexts to be unreferenced locklessly")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170719135957.14603-1-chris@chris-wilson.co.uk


Tested-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>