drm/i915/fbc: __intel_fbc_cleanup_cfb() may be called multiple times

Avoid releasing the same stolen nodes causing a use-after-free and/or
explosions as the self-checks fail, as __intel_fbc_cleanup_cfb() may be
called multiple times during module unload.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200130135136.1878646-1-chris@chris-wilson.co.uk