drm: qxl: Drop device flags attribute

There are no device specific flags that we need to keep track of here.
Let it vanish.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170227204328.18761-2-krisman@collabora.co.uk
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>