drm/i915: consolidate pch pll computations a bit

We need the dpll/fp/fp2 values only when we need a pch pll. So move
them together with the code to acquire such a pll.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>