drm/qxl: allow both PRIV and VRAM placement for QXL_GEM_DOMAIN_SURFACE

qxl surfaces (used for framebuffers and gem objects) can live in both
VRAM and PRIV ttm domains.  Update placement setup to include both.
Put PRIV first in the list so it is preferred, so VRAM will have more
room for objects which must be allocated there.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190118122020.27596-8-kraxel@redhat.com