drm/i915: Don't set up the LVDS if it isn't in the BIOS device table.

We not only check the device type, but also check the addin_offset. If the
addin_offset is zero, it won't be initialized.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
[anholt: hand-applied due to conflicts]