drm/i915: Use IS_HSW_ULT() in HSW CDCLK clock read-out

hsw_get_cdclk_freq() is really just HSW, so we can use IS_HSW_ULT()
instead of IS_ULT() there.

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