drm/i915/gem: Check for a closed context when looking up an engine

Beware that the context may already be closed as we try to lookup an
engine.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/1389


Fixes: 130a95e9 ("drm/i915/gem: Consolidate ctx->engines[] release")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200316161447.18410-1-chris@chris-wilson.co.uk