svga: Fix banded DMA upload

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: 287c94ea498 ("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>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3733>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3733>