drm/qxl: change the way slot is detected

drm/qxl: change the way slot is detected

Instead of relaying on surface type use the actual placement.
This allow to have different placement for a single type of
surface.

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190118122020.27596-5-kraxel@redhat.com
[ kraxel: rebased, adapted to upstream changes ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>