drm/i915: track is_dual_link in intel_lvds

Yeah, all users (both the clock selection special cases and the lvds
pin pair stuff) are still in common code, but this will change.

v2: Rebase on top of Jani Nikula's panel rework.

v3: Incorporate review from Paulo Zanoni:
- s/__is_dual_link_lvds/compute_is_dual_link_lvds
- kill dev_priv->lvds_val
- drop spurious whitespace change

v4: Add a debug printk to display the dual-link status, as suggested
by Paulo Zanoni in review.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com> (v3)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>