drm/vc4: fix an error code

"exec->exec_bo" is NULL at this point so this code returns success.  We
want to return -ENOMEM.

Fixes: d5b1a78a

 ('drm/vc4: Add support for drawing 3D frames.')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Eric Anholt <eric@anholt.net>