i915: lvds panel always blank when booted with lid closed

[Seth Forshee]

When I boot my Thinkpad T410 in a docking station with the lid closed,
the lvds panel remains blank even when this output is active. This
happens up to and including 3.5-rc2.

I've determined that this happens because lvds isn't being initialized
by the bios when I boot this way, and booting with lvds_channel_mode=2
fixes the issue. I see that there's logic in is_dual_link_lvds()
intended to detect this situation, but it's failing because the T410 has
the LVDS_PIPEB_SELECT bit set. The simple patch below fixes my machine
by masking off this bit when determining whether or not lvds was
initialized by the bios.

I'm not sure though whether or not it's correct to expect that this bit
might be set when lvds hasn't been initialized. The alternative seems to
be quirking this machine as is done for some Macbooks. What is the
correct solution?

Thanks,
Seth