drm/qxl: allocate dumb buffers in ram

dumb buffers are shadowed anyway, so there is no need to store them
in device memory.  Use QXL_GEM_DOMAIN_CPU (TTM_PL_SYSTEM) instead.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20210204145712.1531203-11-kraxel@redhat.com