drm/qxl: use QXL_GEM_DOMAIN_SURFACE for shadow bo.

The shadow bo is used as qxl surface, so allocate it as
QXL_GEM_DOMAIN_SURFACE.  Should usually be allocated in
PRIV ttm domain then, so this reduces VRAM memory pressure.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190118122020.27596-9-kraxel@redhat.com