drm/qxl: add qxl_add_mode helper function

Add a helper function to add custom video modes to a connector.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190118122020.27596-22-kraxel@redhat.com