drm/i915/dp: down the DP link even if the reg indicates it's already down

Since the PLL may still be on, and the training pattern may not be
correct.  Fixes suspend/resume on my PCH eDP test system.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[ickle: minor merge conflict and silence the compiler]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>