qxl: read monitors config at boot

Try to read the client monitors config at driver load time, even without
explicit notification.  So in case that info was filled before the driver
loaded and we've missed the notifications because of that the settings
will still be used.

With that place we now have to take care to properly handle a empty client
monitors config, so we don't trip over an uninitialized client monitors
config.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1488363154-6889-4-git-send-email-kraxel@redhat.com