drm/i915/gt: Cancel banned contexts after GT reset

As we started marking the ce->gem_context as NULL on closure, we can no
longer use that to carry closure information. Instead, we can look at
whether the context was killed on closure instead.

Fixes: 130a95e9 ("drm/i915/gem: Consolidate ctx->engines[] release")
Closes: https://gitlab.freedesktop.org/drm/intel/issues/1379


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200304165113.2449213-1-chris@chris-wilson.co.uk