For doing reverese optimus to multiple outputs on a secondary GPU
the GPU can store the blits into a large screen pixmap, unfortunately
this means we need a destination offset into the dirty code, so
add a new API that just adds this interface.
Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Keith Packard <keithp@keithp.com>