drm/i915: fix intel_ddi_get_cdclk_freq for ULT machines

For now, this code is just used by the eDP AUX channel frequency.

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