drm/i915: add intel_lvds->reg

To ditch at least some of the PCH_SPLIT ? PCH_LVDS : LVDS code ...

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

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