drm/i915: enable/disable LVDS port at DPMS time

It would be good to disable the LVDS port when we shut down the panel
to save power.  We haven't done so until now because we had trouble
getting the right LVDS parameters from the BIOS.  I think we're past
that now, so enabling and disabling the port should be safe, though it
would probably be made cleaner with some additional changes to the
display code, where we also bang on the LVDS reg to set the pairing
correctly etc.

Seems to save a bit of power (up to 300mW in my basic wattsup
meter testing).

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>