If the allocation fails the current code returns success.  If
copy_from_user() fails it returns the number of bytes remaining instead
of -EFAULT.

Fixes: d5b1a78a ("drm/vc4: Add support for drawing 3D frames.")
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: NEric Anholt <eric@anholt.net>