drm: qxl: Don't initialize vblank support

qxl don't have support for hardware vblanks so we can't initialize it
here, otherwise we risk getting stuck in drm_wait_one_vblank.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170227204328.18761-4-krisman@collabora.co.uk
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>