commit 552416c1 upstream.

We'd end up NULL pointer dereferencing because we didn't take the
error path out in the parent.  Fixes igt vc4_lookup_fail test.
Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: d5b1a78a ("drm/vc4: Add support for drawing 3D frames.")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>