drm/vc4: Fix oops when userspace hands in a bad BO.

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.")
Cc: stable@vger.kernel.org