drm/qxl: use separate offset spaces for the two slots / ttm memory types.

Without that ttm offsets are not unique, they can refer to objects
in both VRAM and PRIV memory (aka main and surfaces slot).

One of those "why things didn't blow up without this" moments.
Probably offset conflicts are rare enough by pure luck.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190118122020.27596-7-kraxel@redhat.com