drm/i915/bxt: Pass drm_i915_private to DDI PHY, CDCLK helpers

For internal APIs passing dev_priv is preferred to reduce indirections,
so convert over a few DDI PHY, CDCLK helpers.

No functional change.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: David Weinehall <david.weinehall@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459515767-29228-10-git-send-email-imre.deak@intel.com