drm/i915: don't restore LVDS enable state blindly v2

We still rely on a few LVDS bits, but restoring the enable bit can cause
trouble at this point, so don't.

v2: use the right mask to prevent restore (Daniel)
    conditionalize on KMS support (Denial)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>