drm/i915: Set lvds dual channel according to register from vbios

Vbios will set lvds register correctly based on
current algorithm for lingle/dual Channel LVDS when
system boot, so we can accept this configuration
directly, regardless of LVDS enable status.

It fixed freedesktop.org bug #22262

Signed-off-by: Florian Mickler <florian@mickler.org>
Signed-off-by: Eric Anholt <eric@anholt.net>