drm/radeon/kms: fix extended lvds info parsing

On rev <= 1.1 tables, the offset is absolute,
on newer tables, it's relative.

Fixes:
https://bugzilla.redhat.com/show_bug.cgi?id=700326

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>