drm/radeon/kms/atom: fix typo in LVDS panel info parsing

Fixes LVDS issues on some laptops; notably laptops with
2048x1536 panels.

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