drm/i915: don't ironlake_init_pch_refclk() on LPT

This function is used to set the PCH_DREF_CONTROL register, which does
not exist on LPT anymore.

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