drm/gma500/cdv: Check vbt config bits when detecting lvds panels

Some machines have an lvds child device in vbt even though a panel is
not attached. To make detection more reliable we now also check the lvds
config bits available in the vbt.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1665766


Cc: stable@vger.kernel.org
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190416114607.1072-1-patrik.r.jakobsson@gmail.com