drm/i915: Split chv_update_pll() apart

Split chv_update_pll() into two parts ala:
 commit bdd4b6a655749970cc632aafc5fd596c07b60b1c
 Author: Daniel Vetter <daniel.vetter@ffwll.ch>
 Date:   Thu Apr 24 23:55:11 2014 +0200

    drm/i915: Extract vlv_prepare_pll

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