drm/i915: don't save/restore PCH_LVDS on LPT

Because the register does not exist on LPT. The interesting fact is
that reading/writing PCH_LVDS on LPT does *not* give us "unclaimed
register" messages, but the register value is always 0.

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