amdgpu: add padding to the fence to handle ioctl.

I don't think this ioctl is in a Linus release yet.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>