drm/i915: Clean up leftover DPLL and LVDS register choice from pch split.

We used to have these from the product of (pch, non-pch) * (pipe a,
pipe b).  Now we can just use the nice per-pipe reg macros in the
split out crtc_mode_sets.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>