drm/qxl: prefer the monitor config resolution

By default, 1024x768 is the preferred resolution. However, when a
monitor config is given, it should be the only preferred resolution.

Note that the monitor config resolution is passed to
qxl_add_common_modes() to avoid adding a duplicate mode without the
preferred resolution. That would discard the previous monitor config
preferred bit.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>