drm/qxl: allocate small objects top-down

qxl uses small buffer objects for qxl commands.
Allocate them top-down to reduce fragmentation.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20191017132638.9693-6-kraxel@redhat.com