drm/i915: make LVDS fixed mode a preferred mode

The detected fixed panel mode really is preferred, so mark it as such and
add it to the LVDS connector mode list.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@linux.ie>