svga: Fix banded DMA upload

A previous commit ("winsys/svga: Limit the maximum DMA hardware buffer size") made banded DMA transfer kick in when transfering gnome-shell window contents under gnome-shell / wayland. This uncovered a bug where we assumed that banded DMA transfers always occur to the top (y=0) of the surface. Fix this by taking the destination y offset into account.

Cc: 19.2 19.3 20.0 mesa-stable@lists.freedesktop.org Fixes: 287c94ea ("Squashed commit of the following:") Signed-off-by: Thomas Hellstrom thellstrom@vmware.com Reviewed-by: Brian Paul brianp@vmware.com Reviewed-by: Charmaine Lee charmainel@vmware.com Reviewed-by: Roland Scheidegger sroland@vmware.com