drm/i915/gt: Cancel a hung context if already closed

Use the restored ability to check if a context is closed to decide
whether or not to immediately ban the context from further execution
after a hang.

Fixes: be90e344

 ("drm/i915/gt: Cancel banned contexts after GT reset")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200319170707.8262-2-chris@chris-wilson.co.uk