this allows the pixmap dirty helper to be used for reverse optimus,
where the GPU wants to copy from the shared pixmap to its VRAM copy.
[airlied: slave_dst is wrong name now but pointless ABI churn at this point]
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>